What’s the best blue watercolor for the dusk sky?

When I experimented to find my favorite blue for the sky, I focused mainly on the light blue midday sky, with a quick nod to deep dark night skies. These days, I find myself often drawn to the in-between gradient shades of dusk skies: inky night blue at the top, grading to pale muted cyan blue, yellow, or coral at the horizon. What color or color combination is best for this situation?

All of these dusk examples feature dark-to-light blue gradients, with some warm streak on the horizon. The hue can vary from a greenish blue to a violet blue.

Something I love about all these skies: their smoothness. So, I’m avoiding granulating colors like Ultramarine Blue, Cobalt Blue, and Cerulean Blue. Instead, I’m drawn to smooth colors like Indanthrone Blue, Prussian Blue, and various Phthalo Blues (Red Shade, Green Shade, or Turquoise).

Dusk over sea. April 20, 2023.

My first instinct for how to approach these types of skies is with a combination of Indanthrone Blue (PB60) and Phthalo Blue Red Shade (PB15:1), for the darker and lighter blue tones, respectively. In the painting above, for example, I used DV Indanthrene Blue plus Holbein Phthalo Blue Red Shade, grading to a pale mix of ochre and Quin Red for the delicate horizon coral (which I covered over mostly with dark clouds). I was generally satisfied with that approach, but found that I struggled to account for the drying shift in my blue colors: they were much darker when wet.

Drying Shift

In painting these types of skies, I often find that there’s a drying shift in the blue colors that I choose: skies come out duller in chroma and lighter in value than I expected. So I decided to experiment by literally watching paint dry. I tested Daniel Smith’s Indanthrone Blue, Da Vinci’s Indanthrene Blue, and Holbein Phthalo Blue Red Shade.

I took a photo right when I painted out the swatches, and then again about ten minutes later when they were all fully dry. The photos makes it harder to tell the more subtle differences, but as I handwrote on the bottom, I noticed the following:

  • Daniel Smith Indanthrone (left) has less of a drying shift than Da Vinci. Of course, it also starts out duller.
  • Da Vinci Indanthrene (middle) lost both vibrance and value. When wet, it was much greener and brighter than DS, and looked about halfway between DS Indanthrone and the PBRS. After it dried, it was still brighter and greener than DS, but ended up drying closer to DS than it started.
  • Like all PBRS’s I’ve tried, Holbein’s lose both chroma and value and shifted in color temperature. It started out very green, looking fairly similar to PBGS, and dried much more of a middle blue.

Phthalo Blue RS is notorious for having a strong drying shift. Many people dislike it for this reason. Personally, I still love the color when it’s dry – maybe even more so! I think the dry color is a better match for many types of sky than the wet color. So I’m not willing to reject it out of hand. But it is hard to work with for that reason; you have to make a lot of mental adjustments while painting, and/or be willing to be surprised by how it dries.

Here are three more smooth blues: Sennelier Prussian Blue, WN Winsor Blue GS (Phthalo Blue GS), and WN Phthalo Turquoise.

These all appeared to me to have less drying shift. All of them lost a small amount of vibrance and greenness, but they seemed to me to have less drying shift than DV Indanthrene or Phthalo Blue RS. (I did not do this scientifically – this is just my perception.)

Although I was happy with the drying performance of these colors, they all seem to me too green for the color I am going for – at least unmixed.

Lightfastness

I have not done my own lightfastness tests on most of these colors. I did test a version of Prussian Blue (Holbein’s) and found it not very lightfast. Kim Crick has also found that Prussian Blue of all brands are fugitive, though the other pigments mentioned here are said to be lightfast.

Color Mixes

None of these colors alone are quite right, so I made some attempts to find good color mixes from my palette. I looked at two possible strategies: take a color that’s too green, such as Phthalo or Prussian, and warm it up with a violet or magenta; or take a color that’s too violet, i.e. Indanthrone, and cool it down with a greener shade.

Quin Magenta (PR122) + Phthalos

Many of the colors I tested are too green on their own, especially the Phthalos. So I tested Phthalo Blue GS, Phthalo Blue RS, and Phthalo Turquoise mixed with Quin Magenta (PR122) to warm them up into a more violet blue. I varied the amount of Quin Magenta that I added in an unscientific manner: mixing each color that that it “looked about right” when wet. Here are my swatches when dry.

Phthalo Blue GS, Phthalo Blue RS, and Phthalo Turquoise mixed with PR122

Surprisingly, I found that Phthalo Blue GS (left) looked duller when dry compared to Phthalo Blue RS (middle)! This is the opposite to how they typically dry unmixed. I think this is because (1) I added more of the PR122 to Phthalo Blue GS to make it “look right” wet, and (2) the red tones must have cancelled out the green tones to make the mix overall grayer. The same happened with Phthalo Turquoise (left). It dried quite dull. The Phthalo Blue RS mix is the nicest and brightest here. Phthalo Turquoise is my second-favorite because it gets so dark in the masstone, so the contrast makes it look more luminous (even though the tint is quite similar to PBGS if not duller).

One thing I noticed about this combination was that the shade tended to warm as it got toward the horizon. I tried to make even mixes of blue and magenta, but because the blue-green colors were darker, I tended to use more of the them to make the darker shades at the zenith.

This is the opposite of what I want: skies are more violet toward the zenith, and more green toward the horizon. So maybe instead of using a dark blue-green and a light/bright magenta, I ought to have been using a dark violet and a light/bright blue-green (or at least a more diluted one).

Dioxazine Violet + Phthalos

WN Winsor Violet (PV23) + Phthalo Blue GS, RS, Turquoise

This is more like it. The Phthalo Blue GS (left) and RS (middle) are both good, with the GS giving more of that royal blue to cyan gradient that gets greener at the horizon. RS does not get as green and goes to more of a neutral light blue. The Phthalo Turquoise mix is a bit greener and duller, similar to Prussian Blue; it’s suitable for a more subtle sky.

Indanthrene Blue + Phthalos

This is sort of a mix and match strategy: I used DV Indanthrene Blue for the darker/purpler zenith tone, and the greener Phthalo colors to cool it down as it graded to the horizon. As I noted above, this was actually my first instinct for approaching this type of painting before I began these experiments!

DS Indanthrene Blue (PB60) + Phthalo Blue GS, RS, Turquoise

I dropped a spot of water on the RS in the middle there – definitely my error, nothing to do with the paint.

GS (left) and RS (middle) look very similar in the photo but in real life it’s clearer that the GS mix is distinctly greener. Turquoise is even greener and duller.

Compared to Dioxazine Violet mixes, the Indanthrene Blue mixes are a bit more subtle, less intense and a bit less dark. IB also tends to create a bit of texture, at least on this Canson XL student grade paper, whereas the violet remains more inky-smooth. I have to say, although I have historically preferred IB for my palette, I do think Dioxazine Violet mixes better for this particular scenario.

Phthalos + MANS

The Phthalos have impressed me so far, and both work well when mixed with either Dioxazine Violet or Indanthrone Blue to make the zenith more violet. But what about if I want to make the horizon more yellow? A potential liability of either color, but especially Green Shade, is becoming too green in the sky. In dusky skies, I find that one often sees a gradient from blue to yellow without ever really passing through green.

So for my final test, I took the two main Phthalo colors – Green Shade and Red Shade – and compared how they looked when grading to a yellow on the horizon. In order to avoid either one going terribly green, I chose diluted Monte Amiata Natural Sienna (PBr7) as my horizon yellow.

Phthalo Blue GS (left) and RS (right) fading to MANS

Both work! I feel like I’ve seen real-life dusky sky cyans such as those that formed between PBGS and MANS. But PBRS definitely more reliably does that magic sky thing of being blue and yellow at the same time, but never green.

Conclusion

Dusk sky gradients are complex enough that no one color can do them justice. Leaving aside the question of which warm colors to use for sunset horizons or clouds, I think it’s ideal to paint the sky with at least two colors in the blue family: a dark/warm/violet blue for the zenith, and a bright/cool/cyan blue for the midtones.

Zenith Winner: Dioxazine Violet. For the darker, more violet zenith tones, either Dioxazine Violet or Indanthrone Blue work well. Between the two, I tended to prefer Dioxazine Violet for its smoothness, darkness, vibrancy, and lack of drying shift. IB is closer to the right color unmixed, but the violet turns into a lovely deep navy when mixed with your greener blue.

Midtone Winner: Phthalo Blue (GS or RS). For the middle blue tones, grading down from the zenith toward the warmer horizon colors, I liked both Phthalo Blues, Green Shade and Red Shade. (I tended to find Phthalo Turquoise too green, and darker blues like Indanthrone and Prussian too dull.) Unmixed, the Phthalo Blues are arguably too bright, but they look more natural when combined with the zenith color, while retaining the boldness, smoothness, and luminosity I want in the sky.

How to choose which shade of Phthalo Blue to use? Some people simply dislike one or the other (finding Green Shade too garish or Red Shade too dull, or objecting to the drying shift), so if that’s you, then there’s your answer. Use the one you like. If you’re agnostic, or don’t mind swapping between both, I think it depends on your use case. GS might be ideal if you don’t plan on adding any more color to the lower part of the gradient, because the greener shade implies a yellowish horizon glow. RS might be better if you do plan on adding yellow below, since it won’t go as green in the mix.

After all that, I think my original instinct to use Indanthrene Blue and PBRS was fine! I stand by those color choices, and think they can make wonderful colors, even after the drying shift. I just need to work on compensating for it by using them at higher concentrations, making brighter/darker wet mixes, or planning for multiple layers until I get the darkness I want. And, maybe it’s also time to add Dioxazine Violet to my regular palette!