It's a simple fact, when you move from one color space to another color space, that if there's a value contrast you get a strong optical illusion. Strong value contrast can be expressive and dramatic. Like the difference between high or low volume or the low key and the high keys on the piano... Actually, if you're moving from one flat color to another flat color, if there's a difference of color – if one is matte and the other is shiny – that contrast of tactility can keep them visually in the same dimension. It keeps them adjacent – side by side... Another reason is that a matte color and a shiny, transparent color are emotionally different. If something is warm and fuzzy and dense we have a kind of emotional response to that. If something is clear and you can see through it, like yellow or green or red can be, we have a different emotional sensation from that. So there's an expressive difference you can get that gives you more expressive range.