For SLAs, we could go with sunbeam instead of (or in addition to) sunburst. Not many other rainbow themed spells come to mind, but what about some minor SLAs a few times a day each for the different energy types?
Sunbeam sounds better. How about sunbeam at ancient and prismatic spray at great wyrm, both 1/day?
For the adult category, what if we gave it the following ability?
Energy Ray (Sp): As scorching ray, except the rainbow dragon may choose to fire rays of any of the following energy types: acid, cold, electricity, or fire. When its caster level allows more than one ray, the rainbow dragon may choose to fire rays of different energy types.
3/day at adult sounds about right. It's not wickedly powerful, but it doesn't allow a save and can be used in rounds between breath weapon uses. Best of all, as a 2nd-level equivalent, many rainbow dragons can employ Quicken Spell-Like Ability on it.