I think I’ve wrapped my brain around this a bit more, and it has everything to do with the illuminant. I may be regurgitating a few things @Elle has already said, so bear with me.
In order to make any color “warmer”, we need an illuminant that is a warm color, and then we can just “desaturate” our color and it will progressively become “warmer”. Likewise, in order to make any color “cooler”, we need a cool illuminant and then, again, desaturate our color.
I think the specific term is “Excitation Purity”, https://en.wikipedia.org/wiki/Colorfulness. Basically the distance from your arbitrarily set white point, which is your illuminant.
I’m going to add another “picker” thingy to MyPaint so that you can pluck the illuminant from the canvas. My thinking is that if you’re painting a picture you can define your light sources either in the painting or off to one side. Then you can swap illuminants whenever you want and adjust your brushcolor accordingly. We’ll see how that goes.