lego is funny, if you have a pile of red 'stuff' it's hard to pick out the 2x8. however, if you have a pile of 2x8s, it's really easy to pick out the red one.

size/shape is far more useful.

This is a well known idea in data visualisation -- first articulated by Jaques Bertin back in the 60s (I think). Colour is known as a selective visual variable; selective variables are better used to represent categorical data as they "enable us to immediately isolate all correspondences belonging to the same category". i.e. sort your lego by bricks type rather than colour, though sorting by colour will lead to more instagram likes :)

Hmm, when playing with lego, I usually don't know what shapes I need but I often know what color I want.

Yes, but you don't need to sort them by color, because you can easily see which pieces are the color you want.

Yes, color looks nicer, but shape is more useful.