I will ususally ask players to describe their version of any particular spell. In our game we have had Magic Missiles that streak across looking like screaming pink kittens, and a number of rainbow themed spells. As long as there is no mechanical change, the minor variants help the role-play flavor of our game. Casters with a theme seem to work best, rather than a hodge-podge of descriptions like out of the Spell Compendium.
For instance, a Fire Mage may specialize in a number of Flame/Heat related spells, but in addition, most of his spells have Flame-like auras around the area of the spell effect. Read Magic might outline the sigils in glowing ember like script (like LotR). Any particular protection spell might manifest as different colored flames (that don't really burn) around the area of the spell, if there is a visible effect. A strength enhancing spell might have smoke seeping from the muscles/skin of the recipient as the power flows through the individual. Sight or Divinations might be visible through any open flame or the eyes take on a burning eyesocket type look while spell in effect.
There are nearly limitless ways to enhance the spells to fit the casters theme.