What’s the best blue watercolor for the sky?

It happens to be a day with a beautiful blue sky as I write this today, so I can compare my blue paints to the sky in to real life. Which one will win??

Flat Wash Showdown

A blue sky, for reference. Photo by x on Unsplash. (This photo is close to the mid-sky color I’m looking at now!)

Using a combination of blues I have (I love blue) and dot cards, I created this quick side-by-side showdown to compare blue sky colors. What’s your favorite?

Top row:
(1) WN Phthalo Turquoise PB16
(2) DS Prussian Blue PB27 (Granulating)
(3) DS Phthalo Blue Green Shade PB15:3
(4) DS Cobalt Blue PB28

Bottom row:
(5) DS Phthalo Blue Red Shade PB15:6
(6) DS French Ultramarine PB29 (Granulating)
(7) DS Indanthrone Blue PB60
(8) DS Cerulean Blue Chromium PB36 (Granulating)

Any of these might be a good representation of the sky for a particular situation (weather condition, location, time of day, time of year, emotional state). But none of them is exactly the right color for the clear, sunny, winter’s day sky I’m looking at now – at least, not all of the sky. As I look up through my skylight, it seems to me that the zenith of the sky directly overhead is a purple-toned, almost periwinkle. Looking out the window to a section of sky closer to the horizon, it seems to gradually shift to a more cyan hue.

Let’s compare that to the paints:

(1) Phthalo Turquoise is far too green.

(2) Prussian Blue is too dark and grayish. Too moody, not cheerful enough.

(3) Phthalo Blue Green Shade is good for the horizon color, but too green for the zenith.

(4) Cobalt Blue is closest to the color of mid-sky. It has a bit more texture than I would ideally like but is the only one to capture the right slightly red-toned, pastel shade.

(5) Phthalo Blue Red Shade is somewhere between PBGS and Cobalt, significantly less green than PBGS but still slightly too green-toned for most of the sky. Still, it’s good!

(6) French Ultramarine is pretty close hue-wise, maybe a bit too purple, but I really don’t like all that granulation for a sky. I am no longer staunchly anti-granulation in all situations (especially now that I’ve discovered the value of granulating browns for tree bark), but there is something about the texture that just doesn’t feel “sky-like” to me. To me, the non-granulating paints do a better job of capturing the clearness of the sky.

(7) Indanthrone Blue is far too gray and has to be diluted so much to get a light color that damages the paper (at least the Canson XL I’m using now). It might look better on a nicer paper but would still be too muted for this use case.

(8) Cerulean Blue Chromium is pretty and I see why it screams “sky blue” to people, but to me it’s sort of the worst of all worlds in that it’s extremely textured AND far too green.

Flat Wash Winner: Cobalt is the most realistic/closest to the color of the majority of the sky; PBGS is best for the horizon; PBRS splits the difference.

Gradient Showdown

The sky naturally grades. August sky by Billy Idyll, 2023.

Because the sky is not one color (even on a day with a “clear blue sky”), perhaps the best color to represent it is not a single wash but a combination of two blues. I tried a series of quick gradients with some of the warmer blues grading down the Phthalo Blue Green Shade at the horizon. (At the bottom, I did a line of a straight mix of the two shades.)

(1) Phthalo Blue Red Shade to PBGS
(2) Cobalt to PBGS
(3) Indanthrone to PBGS
(4) French Ultramarine to PBGS
(All colors Daniel Smith)

I really like these. The gradients all look a lot more sky-like to me than any of the flat washes.

(1) PBRS + PGBS creates a subtle but very beautiful gradient that I like a lot. The PBRS takes on a sapphire look in the zenith, emphasizing the richness of the blue sky. That said, to me it looks overall too ‘primary blue’ and not purple-toned enough. It looks closer to nostalgic, Polaroid-style photos of the summer sky than the cool real-life winter sky I’m looking at now. Regardless, it gives me a feeling of wonder and peacefulness.

(2) Cobalt + PGBS is the most accurate. It gives me that great periwinkle / candy blue shade of the mid-sky. (It doesn’t get quite purple enough for the zenith.) The granulation that gave me pause in Cobalt alone is softened by the mix. This looks like a very bright, clear, sunny day. It gives me feelings of adventure and optimism.

(3) I did a bad job with the gradient here, but even if you ignore that, Indanthrone is right out (at least for a clear, sunny day.) It is simply too grayish, even if you remove the far too dark stripe at the top. It gives me “storm brewin’ on the horizon” feelings.

(4) French Ultramarine is very striking and cool-looking, but doesn’t feel realistic to me. Even if you cover up the top stripe and assume I started with a more dilute wash, it feels too electric purple, and the granulation is so strong that it still looks too textured to me. Ultramarine (i.e. not French), which has less granulation, might work better. This is a good color for an attention-grabbing, stylized sky.

Gradient Winner: Cobalt to PBGS for actual realism; close runner-up PBRS to PBGS for beauty and “photo” realism.

Glazing Showdown

The sky turns yellow. Photo by Dan Cook on Unsplash

Another thing I tried was glazing blue over a dry, pale wash of my favorite yellow, Schmincke Horadam Pure Yellow (PY154). Often with skies at sunset or sunrise, I want to grade from blue to yellow without going green. What I was curious about was whether color bias and/or granulation would make it easier or harder.

(1) Phthalo Blue Red Shade (neutral, non-granulating)
(2) Cobalt Blue (neutral, granulating)
(3) Indanthrone Blue (purple-toned, non-granulating)
(4) French Ultramarine (purple-toned, granulating)

These all worked out better than I thought they would! None of them turned especially green, which was nice. (Glazing works MUCH better than gradient mixes for this kind of thing.)

(1) I was pleasantly surprised that PBRS worked, as I had the most concerns about this shade simply acting like a transparent blue sheet over the yellow.

(2) I would say that Cobalt is the most beautiful, but it’s a close call because it looks almost identical to the PBRS.

(3) Indanthrone doesn’t go green at all, but is simply looks ugly and gray (perhaps because it is so purple that the yellow is a near-complement.)

(4) Ultramarine looks pretty, but not really realistic for the particular situation I was going for. Again, it’s more stylized. Interestingly, it reads as more blue than gray to me even though it’s even more purple-toned.

Glazing Winner: Cobalt by a nose. Cobalt and PBRS look extremely similar.

Night Sky Showdown

Refresher on what a night sky looks like, since I’m not looking at one. Photo by Jackson Hendry on Unsplash.

Let’s shift from the daytime sky to the night sky. Night skies, like daytime skies, tend to change color of blue from purpler/darker at the zenith to greener/lighter at the horizon. Depending on the time of night, the gradient might be blue-to-cyan (just after dusk) or black-to-blue. The color and intensity of the light on the horizon will also be affected by the ambient light in the area. So, many different types of blue could be useful for different night sky situations.

I tested several of the same blue colors in masstone in a different gradient: neutral tint black, to blue, to a teensy bit of Quin Rose at the bottom to make a bit of a purple horizon glow. I did two layers of each, to ensure I was getting them plenty dark. To be honest, I’m not happy with how any of these turned out (poor gradient work on my part), but you at least get a sense of their capacity as dark night sky colors.

Top row:
(1) Phthalo Blue Red Shade
(2) Cobalt
(3) Indanthrone
(4) French Ultramarine

Bottom row:
(5) Phthalo Blue Green Shade
(6) Prussian Blue

(1) Phthalo Blue Red Shade looks good. It gets nice and dark, grades nicely with the black, and looks reasonably like a night sky navy blue.

(2) Cobalt is horrible. Just horrible. It doesn’t get dark at all and simply ends up looking like a daytime sky with a gray cloud. I guess Cobalt is a daytime specialist.

(3) Indanthrone gets dark, but I was sort of disappointed with its performance here; it looks textured and streaky. Could be user error. I usually find it very nice as a dark shadow color, and its hue is pretty close to what I was going for.

(4) French Ultramarine has a luminosity that I’m really digging. In this instance the granulation isn’t bothering me because it suggests distant stars. The purple tone feels right for a night sky.

(5) It doesn’t really show in this photo, but PBGS is much greener than PBRS, as well as more saturated and glowing. It looks like the night sky by the horizon right after sunset, when there’s a still a lingering tinge of yellow. Again, I would use this for a horizon color but not really for the whole-sky color.

(6) Prussian Blue gets the most satisfyingly dark. Although granulating (technically), it appears less granulating than Indanthrone, and has good coverage. It looks very similar to PBRS but with a slightly more greenish tone – between PBGS and PBRS in tone, but darker than either.

Night Sky Winner: I just love the Ultramarine here! PBRS and Prussian are runners-up.

So what sky blues should I have in my palette?

The short answer is “it depends”! There are so many different sky conditions and it doesn’t need to be accurate anyway, so you should probably just choose whichever blues you want. You can make anything work. I do not think it is crucial to have a specific paint color (especially one you don’t otherwise like or have another use for) just to paint the sky. Different skies will look different anyway! The hubris of writing this piece is that the days I edited it, there were totally different-looking skies.

With that out of the way, my takeaway from these experiments from these experiments was a new appreciation for Phthalo Blue Red Shade! I’ve always just thought of it as PBGS’s less-useful cousin, but I was really impressed by how it did in these tests. It got a solid “B” or higher in every situation. It worked great alone or in a gradient, day or night.

If that’s not your favorite blue, though, it’s not necessarily a must-have. Personally, I most enjoyed the Cobalt-to-PBGS gradient. I’ll be making that one again. And for night skies, I’ll be exploring Ultramarine and Prussian Blue.

The best color blue for the sky? Any one you’ll use and enjoy having on your palette!

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