You can buy inexpensive PWM controllers all day long on Ebay for motors and such. They put out 12v; run some LEDs in series to get to the right voltage or use current limiting resistors. Use a $20 controller for each color with a corresponding knob/potentiometer. I'm sure some of the companies that make hobby electronics kits (formerly a market dominated by heathkit) would be glad to make up a circuit board for the lamp assembly and sell the kits with parts that someone solders together themself. One can also outsource every part the manufacturing if you wanted to sell ready-to-go systems. Hire a company to make metal or plastic adaptor plates, hire someone to build and assemble the circuit boards, etc...