It was a forehead smacking moment. It took me much longer than I really want to admit in this eternal forum of public-facing internet to realize that the letters in pigment names stood for colors. PB meant "Pigment Blue," PR was "Pigment Red," and so on. But that slow dawning of realization also meant that I will never, ever forget what those letters on my tubes of watercolor paint mean.
All watercolor paint is made from powdered pigment, a binder (typically gum arabic), additives and/or preservatives, and water. You can buy the individual parts and mix your own watercolor paint if you so desire...and trust me, I've started looking into it more than once. There are some mesmerizing Instagram accounts from people who make handmade watercolor paint.
Understanding the watercolor pigment numbers on the labels isn't really necessary to be an excellent watercolor painter, but I'm a nerd so I enjoy analyzing them anyway. Paying attention to them makes it easier to shop between brands. It's also helpful to keep an eye on when mixing paint, since the more colors you use the more likely you'll make mud. Any paint that's made up of more than one pigment is secretly going to mess with your mix if you aren't paying attention to those little codes.
In my nerdy color explorations, I gathered up the tubes of paint in my collection that are made up of the exact same pigment, but named differently by the manufacturer. Then I made little swatches of them in my handmade watercolor sketchbook, to see how the binders affect them. Here's an example from my five (5! I'm ridiculous) brands of PB29. (And my blooper. Don't be like me kids--get fresh paint from the tube instead of assuming that dried blob on your palette is the color you want for this incredibly non-scientific experiment.)
Many brands call the pigment PB29 "Ultramarine Blue" but it also goes by French Ultramarine, or Ultramarine Deep, or whatever else the marketing team came up with. While this one is fairly easy to recognize since they still all have the word "ultramarine" in their name, we savvy watercolor nerds know that it's the pigment number that matters.
My beloved Indigo is an example of why the pigment combos matter more than the name. I'm obsessed with this color, so again I have 5 different versions of it from multiple brands. Everybody calls it "Indigo," but it's always a blend of pigments, not a single color, so each one varies greatly.
The Shinhan blend is the extreme outlier here, including no black at all but green instead. Weirdly, it also paints more like a black than some others that lean more blue. In this case, I'm actually quite pleased to have so many different "Indigos" because of the variety. Sometimes a black leaning Indigo just won't cut it.
If you want to learn more about watercolor pigments, Handprint is an exhaustive resource, although I totally fan girl over Tonya's deep dives into individual colors at Scratchmade Journal. Now go forth and paint with more knowledge!