Is Prismite considered colorless or all colors for the purpose of Brawl/Commander decks?
It's colorless:


  903.4. The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card’s mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).


Prismite's ability says: "{2}: Add one mana of any color." and not "{2}: Add {W}, {U}, {B}, {R} or {G}." No mana symbols appear in the rules text, so it's colorless.

Note that Brawl uses the same definition of 'color identity' as Commander:


  903.11a Brawl is an option for a different style of Commander game. Brawl games use the normal rules for the Commander variant with the following modifications.