KirAsh4:
Not sure what project you're doing, not sure what LED you're using. However, most RGB LEDs have different forward voltage values, so you can't use the same size resistors on all three channels. And if you are using a single resistor on the cathode, you're doing it wrong.
Here's the spec sheet for the RGB LED:
Forward voltages are 1.95/3.3/3.3 respectively.
As for the number and size of the resistors, going from memory (don't have the book in front of me at the moment) I think the
resistor strengths are all the same. Maybe someone that has the book handy can check.