So, what's the solution? Mix your own! Deb will take us through the essentials of that in this video:
The basic premise behind what Deb is doing is using complementary colours to neutralize to black. Complementary colours are colours that are directly opposite on the RYB colour wheel:
Now, of course, paint pigments are not perfectly pure and so the complementary scheme is a start point in the determination of what to choose to mix from. So, for example, if you mix a red that leans to the violet side, then the green you'd use to neutralize should lean to the yellow. It will take some experimentation, but pay attention to the pigments in the paint as well, this will help for consistent mixing across brands.
Also, the base colours you choose for this is handy for when using the mixed black for shadows. For example, in Deb's first mix, she had a largely blue base and so the hint of blue will come through in the black when thinned out and that can make for realistic shadow effects on lighter surfaces, such as snow.