You don't need large power resistors - just parallel some regular ones - if you use 400mW resistors then the red LED needs 2 resistors (either 2.7 ohms in series or 12ohms in parallel). green / blue would just about be OK on single 400mW resistor for light use (not continuous duty).
However the resistors will get hot (hot enough to burn you even), need to be spread out and need free airflow to cool them (not inside a small box for instance).
For 250mW resistors you'd need 3 for red, 2 each for blue/green.