I have found that the best results so far require a diffusion layer very close to the LEDs and another just above the negative. With just one or the other, I see obvious patches of red, green and blue light. With both diffusers I see a very even white light over the whole of the open area which is actually 5" x 7" despite this being marked as a 5" x 4" enlarger.
I'm not sure if my PCB is from a PAR 56 or PAR 64 light but mine also has 153 LEDs and looks very similar to yours, if not identical.
What is your distance from PCB to negative?