I’ve spent quite a bit of time in designing a full 16x16+X BLM.   It’s using the ADAfruit silicone pads and rear-mount LEDs. First I tested the LED concept with OSHpark boards. I tried red, green (not yellowy green), yellow and blue, and all of their combinations for the “third colour.†Although I was happy with the illumination for red and green separately, their mixing is unfortunately quite poor. What does work however is red+blue = pink and green + blue = cyan/white. The latter is