Nice to see Mr Gurney here. The man is a living legend.

Another way to convey the same is that the colour vision of a typical human can be described using three independent variables. And the set of variables you use defines some colour model: RYB, RGB, CMY, HSL, HSV, etc.

Yes, you can convert from one into another. It should be always mathematically viable, even if in some cases it doesn’t make real life sense; for example, if you convert RGB #00FF00 = (0,1,0) into RYB, you’ll get a negative amount of red.

When you’re painting it isn’t a problem, though - as the author says, artists don’t often use saturated colours. And sometimes they also distort palette on purpose, picking some other colours as their primaries, to achieve some effect.