Alright, the "T" key now colours a cell background pink (and back to white).

I also made it so that a "puzzle restart" clears the pink colours (this wasn't happening before).

Thank you so much!

I didn't think about it at the time, but now that I think about it it's a big advantage for t (relative to the comma) to be left-hand primarily. That matches up with, I assume, most people using left hand for numbers and right hand for arrow keys or mouse.