I saw two threads here earlier about how the Legions have their roles (naturally) and some contingency built-in with the Primarchs that they could replace each-other or something. Since then I've been thinking wouldn't it possible to make some logic deduction as to what those Legions and Primarchs would be doing? Even if
GW didn't intend it?
Also what colors would they have?

Based on the pre-Heresy colors we've got roughly 4 white, 3 black, 3 grey, 2 purple, 2 blue, 2 red, 1 yellow and 1 green. If the main colors would all be made of two groups with the same number it could "logically" be black and purple for the two lost Legions (e.g. 4 white, 4 black, 3 gray, 3 purple, 2 blue, 2 red, 1 yellow and 1 green) or any combination of one black, one grey, one purple, one blue and one red