So my friends and I were testing Color Spray in the tavern. We discovered that the formula is 10+spell lvl (starts as 1)+Int modifier+Evocation bonuses. When heighten is used it's based on the lvl of the character, NOT the highest casting ability from any class (i.e. a lvl 17 that has heighten will bring it up to a lvl spell DC value of 9 regardless of the highest spell lvl that character can cast). My friend's split was 9/7/4 Drd/Wiz/Rog, with a 13 for the Int modifier. The base DC was 24. When heightened, it went up to 32 (the 8 DC increase coming from the fact that it's considered a lvl 1 spell that is increased to a lvl 9 spell). Evocation, Insightful Evocation and Spell Focus items increased the DC. Illusion, Enchantment, and Conjuration DID NOT.
Hope this helps people with trying to improve their Color Spray DCs and/or helps the Devs fix the problem