Xachi, Sunburst Sovereign

Prismatic (Whenever you cast a spell that's two or more colors, put a +1/+1 counter on this creature.)
As long as Xachi, Sunburst Sovereign has three or more +1/+1 counters on it, draw an additional card during your draw step.
As long as Xachi, Sunburst Sovereign has five or more +1/+1 counters on it, spells you cast cost {2} less to cast and can't be countered.