If it goes by Final Fantasy standards set by FF12, White is Curative, Black is Elemental, Blue is Monster Skills, Red is White/Black usually limited, and Green is Status Effects like Blind and Poison.
If this is a Legion Saga fan-remake, why don't you use the Orb system from LSII and LSIII? I know there is a script for RPGVX at least that allows you have skill-learning equipment; pretty sure there would be one for Ace.