It's definitely possible. I'm not sure why it isn't working for you. Maybe post a picture of your object editor so we can see.
I definitely am using this in my own map somewhere. You can't use custom units, only models. The models are forced to be a certain size (I think it is 1.00, but maybe it changes if you have the "scale projectiles" set to true on the caster) and I think they will always have 255, 255, 255 for their colours.