The different coloured gems are very characterful, however the below alternative is a more simplified idea using the existing mechanisms.
As well as colour/element definition each "school" could have a role. Initial thoughts would be
Black, necromancy - spells to raise undead, cure damage on undead, boost performance of undead, fear causing spells,
Brown, Earth - spells of a defensive nature, increase save of a unit, raise earth walls eg make a square walled,
Red, fire - offensive fire spells, fireballs to damage units like artillery, fire wall to line one edge of a square causing damage to units crossing it,
Blue, water - reduces movement by swamping up squares, can part water to let units cross lakes and rivers,
White, air - facilitates movement by improving move of a unit, allowing an extra turn, allowing a unit to activate without drawing a card (if a spell fails it stops the wizard casting another but as the unit hasn't stopped then the command activation isn't ended)
Each spell could have a separate activation number depending on the school of the caster and the rank/level of the spell which might be written as follows
Wings of Wind: (white school) turns a unit in its own square for free does not count as the unit being activated
White 2, Red 3, Blue 3, Black 4, Blue 4
Raise Dead (Black school) cast on a square where a living unit has been destroyed (note necromancers need to put a skull/skeleton markers on squares where units of living troops are destroyed) if successful place a zombie unit in that square providing the square is vacant or only has a single normal size friendly unit in it.
Black 3, white 5, blue 5, brown 5, red 5
The calculations are that black spells are always +2 for all other schools, all schools find black difficult at +2 likewise black finds all non black spells difficult so +2.
White is difficult for brown and vice versa s +2 for difficult
Red is difficult for blue and vice versa so +2 for difficult
If a spell is not your school or difficult then it's +1
A wizard can cast as many spells a turn as he wants and like normal activations needs a higher draw each time to make it successful. If a draw of 10 is made ON A SECOND SPELL ATTEMPT IN ONE TURN then the wizard is injured or hors de combat. That way a cautious wizard casting once a turn is safe but an insane magic mad wizard may be taken by the demons.