Originally Posted by kranu
If you're feeling fancy maybe you could use an analog pot (and include a way to turn it more easily). That way, you don't have to stick with just two colors!
It'd be more work but doing PWM will give you better control over brightness and waste less power than just putting in a variable resistor. One of the tiny 6-8 pin uCs like a PIC with a small FET would be more than capable of doing this.