@b0rk I mean, there's a point where a metaphor doesn't work anymore, because it's not the truth, just a way to explain part of the truth. If you need perfect accuracy you're just explaining reality.
The paint metaphor also breaks down because there are tools for recreating paint colours accurately from pure pigments. Do that for (sxa), (sxb), and s, and you recover a and b. So it's not really suitable as a form of key exchange. But it is easy to understand on a basic level.