
A very strong and vibrant semi-opaque reddish orange, PO73 oranges such as Winsor Orange (Red Shade), pictured, can be substituted for a warm orange-red in color mixes.
(I am calling this in general “Pyrrol Orange” because every brand seems to want to claim it for their own.)
Experiment Results
I found Winsor Orange (Red Shade) extremely enjoyable to swatch out. Very pleasant handling.
Gradient: Look at that. It’s a thing of beauty. One of the smoothest gradients I’ve ever seen. In mass, it’s a deep bold vibrant orange, and in dilute it is a coral peach.
Opacity: This is an interesting one. WN describes this as semi-opaque and it feels like it does have that kind of pop and heft you expect of a semi-opaque, but I didn’t detect much if any color on the black line. I would say this is significantly more transparent than, say, Pyrrol Scarlet (PR255).
Glazing: Glazes to almost a neon red.
Comparison to Other Colors
Winsor & Newton – Transparent Orange

Transparent Orange (PO71/DPP) can vary in color a bit from brand to brabd, but generally they are less red, more muted, and above all far more transparent than PO73 oranges.
Da Vinci – Vermilion Hue
I found Da Vinci’s Vermilion Hue, which is a mix of Vermilion (PR188) and Perinone Orange (PO62), very similar to this color but a but more muted.

Comparison to Other Brands
Da Vinci – Da Vinci Orange
Another PO73 that’s very similar.

I couldn’t tell much difference between these. In person, DV looks a bit more middle-orange and less red-toned than the WORS, but it’s subtle. They were both lovely to swatch out.
Commercial Mixes from This Color
Holbein – Brilliant Orange (PO73, PY110)

This is a bright middle orange made from a mix of PO73 with Isoindolinone Yellow Deep (PY110). Brighter than it appears in the scan, this is a vivid, bold, Halloween orange. It’s a bit “on the nose” for me but it’s great if you want a really orangey orange.
Holbein – Permanent Yellow Orange (PO73, PY74, PY83)

This is a bright, vibrant yellow-orange made from PO73 plus PY74 (Brilliant Hansa Yellow) and PY83 (Diarylide Yellow). In the same general family as Yellow Orange (PY110), New Gamboge, or Hansa Yellow Deep (PY65), but a bit more orange with a deeper masstone.
Holbein – Shell Pink (PO73, PW6)

Holbein’s Shell Pink is an opaque pastel peach/pink made from PO73 + Titanium White.
Holbein – Vermilion Hue (PO73, PR254, PY110)

This is a tomato scarlet, a bit more orange than Pyrrol Scarlet or Naphthol Scarlet, made from Pyrrol Red plus the two pigments that make up Brilliant Orange (above). I like the way this grades from scarlet to peach, though I’m not sure I’d pick it over a single-pigment scarlet. It is similar to Da Vinci’s Vermilion Hue, which is made from different pigments (see it on the page for Naphthol Scarlet).
What Others Say About PO73
I have very high regard for this pigment; it is everything modern pigment chemistry should be. Provided you use the transparent single pigment brands, this makes a very versatile and reliable paint, worth trying for florals and other brilliant painting styles, and splendid as a warm, almost pinkish tint or blush color for caucasian flesh tones. It is an ideal red orange pigment for the six paint secondary palette.
Bruce MacEvoy, handprint.com
My Review of Pyrrol Orange
I nabbed this color on the strength of enjoying it in the Winsor Newton Dot Cards. It’s super-vibrant and has wonderful handling.
It’s a fun color in “cliché” sunsets.

It’s also a useful color for orange sandstone desert milieus, where it mixes wonderfully sun-drenched canyons.

As a bold semi-opaque, it’s useful anytime you need an orangey pop color. It also mixes up lovely peachy pinks or yellow-oranges (as evidenced by the existence of Shell Pink and Permanent Yellow Orange, both of which are easily mixed on your own with white and middle yellow respectively).
In dilute, it’s very peachy, as in the midtones of this blue-to-orange sunset. (For the yellow tone at the bottom, I mixed in MANS. One thing about this orange is it’s not yellowy.)

I struggle to find a place for this color in my palette – it has a similar role to scarlet, but I don’t find it quite as useful. I keep expecting PO73 to neutralize blues and greens better than a red would, but, oddly for an orange, it has a tendency to go purplish. I find the purple-and-orange-at-the-same-time mixes it makes with blue to be unique, odd, and kind of nauseating, perhaps because it reminds me of the color of bright sunshine against my eyes when they’re closed (a feeling I associate with going for long car rides as a kid). But I guess that’s a “me” problem.
On my palette? No, for personal reasons.
Favorite version: Winsor Orange (Red Shade) or Da Vinci Orange.