Everyone seems to have magic in some level, well at least if they are a unicorn or someone close enough to the main characters. That said it seems that the color of their magic shows a magical effect and color that is individualistic for the person or pony, at least to an extent. Meaning you can tell, if you pay attention, whose magic it is just by looking at the magical aura.
Is it because of their cutie mark, the way they were raised? their parents and family history? So many possibilities and so many things it could mean. Are they only colored differently or is there more to it like a finger print?
So take 30 minutes to draw something out that shows how this might work and why it might be for their world. Then you will have 15 minutes to send it into the inbox.