In most games magic is split like:
red(fire/chaos/offense) - direct fire damage, damage over time, defense and resistance affecting debuffs, volcanic manipulation
blue(water/holy/order/resistance) - ice/water damage, healing, mind affecting spells, elemental protection buffs, sea/water manipulation
yellow(air/electricity/speed) - random electric damage (lightning is unstable), stun, speed affecting buffs, weather/wind manipulation
green(earth/nature/defense) - physical damage, nature summons, defensive buffs, speed affecting debuffs, earth/terrain manipulation
Also there should be cross-domain spells. For example Volcano should be a fire/earth spell. while holy shock(massive lightning damage to undead) should be water/air.