Cool! I love mood lighting. What sort of 'stuff' will go over the LEDs? Are you going to put something like rippled glass or odd shapes on it; maybe motor-driven? Those round glass globes that go over cheap indoor light fixtures work very well and look pretty cool too.
I'm adding this to my (long and growing) list of 'want to do' Arduino projects.
I noticed you have resistors on the MOSFET gates and also in line with the LEDs. You're sort of mixing two approaches here, and your're not really driving the MOSFETs properly. I'd suggest the following for more reliability.
The gate resistors on the MOSFETs should tie the gate to the source and should be a high value such as 10k-100k. The Arduino pin will drive the gate directly. This improves the response of the MOSFET by draining off the capacitive charge on the gate when the pin goes low, and allows the pin to go high very quickly too. The gate is a very high impedance, so it will not draw any noticeable current from the pin. (in fact you may be able to eliminate that resistor completely)
The resistors in line with the LEDs should be a wattage and value that limits the current to a suitable level, as if the MOSFETs didn't exist. That way the resistors are taking the brunt of the heat and the MOSFETs will run cooler because they are just switching fully on or off, rather than acting like a throttle.
The wattage of the resistors is calculated
(PS volts - LED volts) * LED amps
The ohms is calculated
(PS volts - LED volts) / LED amps
So for example, if the LEDs were 350mA (.35A) and 3.6V each and your power supply is 12V, then:
Ohms: (12 - 3.6) / .35 = 24 ohms each
Watts: (12 - 3.6) * .35 = 2.94 watts each
This will cover for the LEDs running at 100%. If they only ever get to 75% brightness, then multiply the watts by 75%. But the ohms stays the same.
Cheers!