Sorry, I didn't notice your last edit until now.
Originally Posted by BlackRose
What is the effect this system can have when trying to modify the vertex colour of a unit to only one player? For example, making a unit's alpha zero for all players except one.
In that case, GetUnitVertexColor would return different values for different players, which could cause a desync depending on what you do with those values; however, since you are likely to only use them for graphical stuff, this shouldn't be a problem.