It can be wonderfully useful to just skip over mixing certain colors – it will save you time and paint! … Various shades of green are pretty easy to mix up, but especially if you are a landscape or botanical painter (or enjoy painting frogs eating pickles on pool tables), you’ll find that mixing up that much Green will quickly run you through your primaries, and can leave you spending more time mixing your colors than painting with them!
Further, having some “base” convenience colors will allow you to make quick adjustments with small amounts of your primary (or other) colors.
Jess Greenleaf, Creating Your Perfect Palette
A convenience colour I have come to regard is orange. A single-pigment orange as this mixes with blues to give all sorts of browns and greys. Using yellow + red to mix orange doesn’t give me the results I want when mixed with blue because the yellow then leans the mixture into the green part of colour mixing space.