Very good idea,
I like the idea of customising the spells and creating new ones but how it will be done exactly is another thing as there are many ways it could be done.
Perhaps each school of magic is related to a certain thing, ie the red magic casts spells which cause fire and burning, the blue causes healing etc, crystal can be primary attack spells and brown could be non combat related such as alchemy, herblaw etc.
If you have lets say a high level in red spells you could combine a certain feature in it to do with fire with crystal to make a primary attack spell which has an effect of fire with it, or something else.
Each spell could consist of about 5 attributes,
Healing:
Power:
Range:
Elemental:
Destruction:
You can use different attributes the higher your magic is in a certain type (crystal, red, brown) to make a unique spell combining all of them.