A robust box with a large RGB led (could be a separate dome that lights up, or make the toy a cube that lights up entirely?)

There would be 3 knobs to allow for mixing the colours - the child could then adjust the colour at will

Chunky button(s) to change light up mode:

  • flash
  • pulse
  • fade in/out

Make it have a (USB?) rechargeable battery, with auto-off.