Different physical materials can be modified using different colors of light.
Metal (Red), Stone (Orange), Liquid(Yellow), Organics(Green).
Violet modifies the existing light, preventing it from being affected by others who use the magic, or enabling some external trigger to modify the element using the other colors of light (Like triggering a sudden pressure change in the air to push something onto somebody, or metal to shape around someone to form restraints).
Blue modifies gases, but can also function to modify weight of an object. It wouldn't lower the density but think like a small bubble of less dense gas surrounding an object pulling it up (Think helium balloon pulling something up by a string).
Not sure if that helps, but I'm thinking of other cool gadgets and crafts and such that can be made (Repeating hand crossbow, dart shooting things, mechanical calculator, stuff like that)