I am making a DIY LED light and need to know if it is possible to control four different colors of the LEDs and have them go from 0 to 100% and back down to 0% to resemble the sun.
3x120 = 360 Watts of LED lighting... Is that a typo, or are you trying to blind your fish? 
High-power LEDs (1W or more) require a special constant-current power supply, which you can buy or build. Yes, you can get (or build) dimmable power supplies.
Great. I have the dimmable drivers for all of the LEDs, how do I go about building a controller with a clock to control the light? Any info will be greatly appreciated.
Opps, I just noticed the 1st part of the response. The light is for my 135 gallon tank.
It is 6 feet long, 2 feet deep, and a foot and a 1/2 wide. For SPS corals that is not too much light.
BUMP
What driver(s) are you using?
I've got 56x3W dimmable leds on my nano right now 
I'm using PWM for brightness control on a Mega 2560 with a color display, real time clock and Wii-Chuck for keyboard entry, internet hookup and bluetooth for my android phone
I am stepping my display up to a full iphone size touch-screen (ebay #270972145966) so can do away with Wii-Chuck as I haven't finished Android programming yet and screen seemed more bang for the buck programming wise
I'm running White, Blue, Red, Green, UVA, Aqua and 1W IR
What colors are you running (I assume white, blue, red, green)? BE CAREFUL on color mixing, you miht end up with a 8K/10K washout color... remember White led's are primarily made at the factory using a yellow led doped with a little blue... think thats the way it is anyways)
Hope you're not wasting led power by lighting your overflow(s)!
Are you using any focusing lenses?
Are you over-driving any of the LED's so you can simulate lightning?
Are you running common-anode, common-cathode or individual drive?
How many led's in your strings?
If you're running strings of led's, are you using PLED devices?
Do you have a PAR meter (you might want to pick one up/borrow for short time)
Do you have an Apex or ReefKeeper (you could use either of those for trigger start and/or physical brighness of leds)
Where are you located?
PM me for the link of what my light looks like