It’s just bad intuition, sorry :) E.g. when you mix two grays (#80808000) in RGBA space you probably don’t expect white. None of R, G, B, A are additive in a s

It’s just bad intuition, sorry :) E.g. when you mix two grays (#80808000) in RGBA space you probably don’t expect white. None of R, G, B, A are additive in a sense of color/transparency mixing. They are only additive component-wise regarding the resulting color, i.e. R+G+B is a color, but two reds aren’t 2R.