Seems perfectly reasonable, there are lots of greens in Nature, but few purples. Orange is a yellow-red with little saturation/chroma cost, and often used as a low-chroma earth-color brown, tan or ochre.

…But, depending on the colorwheel model yellow and ultramarine blue can be 180-degrees apart so mixing a high-chroma green can be near-impossible due to saturation/chroma costs of two pigments that-far apart in Hue. Whereas a high-chroma “convenience green” spaced between yellow and blue provides flexibility and greatly-reduced saturation/chroma costs. Greenish yellows, yellowish greens, bluish greens and greenish blues in high and low chromas are then available. And mixing a neutral from green and red is easier.

Personally, I have rather have a burnt umber or burnt seinna rather than the white in watercolor 6-color set. White in watercolor set is redundant, in a gouache or casein set it’s more useful. In acrylic, I want the white but forgoe the black for the BU or BS instead.

YMMV with use.

You might find this Handprint article about the historical choice of this palette interesting: https://www.handprint.com/HP/WCL/palette4d.html

The idea of exactly three primary colors is somewhat arbitrary. (Yes, there is some interesting stuff about eye receptors and mixing curves, but it’s not as clear-cut as people think, and the three-color theory was laid down when the available pigments were a lot more limited.)

Those four colors are nicely spaced around the spectrum, and even people who subscribe to the three primaries are more likely to keep a convenience green in their fifth slot (after three primaries and an earth tone).

Edits: By convenience green I don’t mean a multi-pigment blend, I mean they call anything but a primary a convenience color. Personally, since I paint abstracts and not landscapes, my fifth color on any limited palette is violet. But it’s an unusual choice.

I agree with Pinguino, this is mostly about marketing to the demographic most likely to buy the product.

That said, going back to the idea of color theory, RYBG colors are the opposition theory colors. So beyond just being good marketing, it also has roots in how we perceive color as well, red/green – blue/yellow oppositions are commonly seen opposites. They function well for a basic palette, though I would prefer having a Magenta in my basic list of colors. Cyan-Magenta-Yellow-Red are my core 4 (plus white when working with opaque media).

I too tend to suspect the inclusion of green in paint sets is largely due to the perceived need/usefulness of it, especially for landscapes. A few other, at-least-as-good reasons to include green in your palette:

•green + red or magenta makes some of the darkest possible mixed colors & blacks, if both are dark to begin with (no white added)

•a high-chorma green can be a base color from which other, more natural greens are made, especially by adding yellow-orange, orange, brown, or even a bit of red. (counterpoint: if natural greens are your goal, not using any tube green at all might be a better idea – some artists might be able to go a lifetime without needing a tubed green)

•a tubed Phthalo green cannot be matched in chroma by any yellow + cyan (or blue) mixture. Sometimes you just need/want the highest-chroma green, turquoise, or vivid lime green…even for realism

•as stated here, red-green-yellow-blue opponent colors is possibly a better framework for describing how we perceive color in the world than a 3-primary framework. This also allows having 2 complementary (or near-complementary) mixing pairs, versus zero for most 3-color ryb/cmy palettes (although some red + blue combos can get kinda close to black)

Why not include orange or purple? Oranges can be mixed to a decently high chroma with a middle yellow + middle red. High-chroma purples are not common in nature (other than flowers), but it’s a color I sometimes have a need for – just like high-chroma greens.

Why green? What do y’all do with the green? And if green, then why not purple or orange? Thanks!

The short answer, which is also the strangest: People have trouble mixing green. Not kidding.

I’d put a good brown instead of a green, if I owned a paint company.