Interfacing with a string of 12v RGB LEDs?

I have a string of RGB LEDs wired for 12 volts that I'm currently controlling with this controller:

I've played with other controllers such as this one:

Its ok, but I'd like more control over the fading modes, which is where the Arduino would be nice. I searched around for "arduino led controller shield" and things like that and didn't find anything particularly useful. I'm wondering if there might be some LED controller that people know of that can easily interface with the Arduino?

My main goes is to get more control over the fading modes. For example, to be able to fade very slowly between blue and red and back.

Thanks for any help.