What's the possibility of a design that the average person with good technical and building skills can take and actually make an LED head for an enlarger right in their own shop? Has the home application to a variable contrast head come along far enough to make one that works well?
Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand
It's all fairly simple stuff which you can make as complicated as you like. You can have a micro-controller sorting out your blue/green ratios via PWM drivers if you want or you can just have some switches. I prefer the simple approach and the most complicated thing I will do is build the single control variable contrast PWM controller. But it will be with discrete components rather than a micro-controller.
The PCB from the stage light which I used is easy to get out, has an illuminated diameter of about 5" and can be used as is in an enlarger. Ignore the DMX controller and the PCB just has four connections: red, green and blue. just add a power supply and it is ready to go.
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...