An incorporeal golem? That's new. Pity it has 20 RHD, though.
The prismatic golem relies on so-called 'prismatic touches' to attack, which are... not good. In 75% of the cases, it'll end up dealing 5d6 damage, with occasional Insanity (one of the worst effects as far as save-or-loses go) or 10d6 damage instead. With only two of these touches a turn, the golem's full attack damage will probably be far beneath what the party rogue could output with one attack.
Prismatic Body blinds creatures with, hold on, less than 8 RHD, making it all but useless in a level 20 party. The second feature punishes monsters for touching you, but what little offensive pressure you can put out won't really incentive them to go do that. Finally, it lights up your surroundings, which is probably the smallest issue to worry about at this level.
Magic Immunity is nothing surprising: prismatic spells heal the golem and are also ended by it, other magic is ineffective.
All things considered, this is a very interesting golem that ultimately just sucks. -0 LA.