[muxxa]

There is a big problem in that the purple is much less visible when it appears on top of your own piece vs. a free square. I would suggest instead to color code it red when one of your own pieces is being attacked. There is no need to show the squares your pieces are attacking IMO (yellow heatmap)

Why not take this further to count up the simple point value of attacking/defending pieces, so e.g. a pawn that is attacked by a pawn but defended by a pawn won't show red, whereas an attacked but undefended knight would show '-3 redness'. Also if this was just briefly 'flashed' after the opponents move, or after you reopen a browser tab, it might serve as a nice reminder of the positional situation without interfering with regular/plain board visualisation, and without making the player reliant on it.