Interesting concept, I quite like it!
I found it quite easy to select each of the base colours - though took me a few tries to realise I couldn't recolour from blue to yellow without pressing W first. Mixing colours made it a lot more difficult, and I was struggling to get the colours right without dying lots first.
I like the pieces that made each level more challenging - the bars that stop you from progressing if you're a certain colour, the disappearing platforms, and the black hole that chases you a few levels in.
I think one thing that might benefit is having a button for Retrying a level after you die - it was a bit of hassle to click my mouse each time after I got my hand in a comfortable position to press J, K and L.
Overall, I enjoyed playing it! Very nice game!