Back in post #10 I gave you an example of how I PWM drive a RGB LED. There are any of several ways to go about it. Note the resistor values used in series resistors. Note how the Green and Blue resistors are not the same as the Red.
A simple Google of LED MOSFET Circuits will net you a few dozen circuits.
No there is no definitive answer. None I am aware of anyway. Even if I did want a constant current source I would not use an LM317. If I really did want a constant current source I would use a newer more efficient better chip solution.
Ron