Well, I have t tried it, but there may be a default VCE variable replacer.
Try this:
[0] [x] [0] [Onactivate] [Self] [VCE_Ifvalue] [<var:br:activate>] [==] [1] [1 1]
[1] [x] [0] [Onvariabletrue] [Self] [Setcolor] [Any other color besides the current color of the brick]
If it turns the other color, then use that variable replacer. If not, then replace the (br) in the example to (cl) or (pl), either of those may work. None of these may work, in which case we need this, as it would be another good VR.
For other keys, it couldn't work, as the key action is acting upon the player, not any brick. However, V21 made your paint key activate, just like default left mouse button, as well, in which case it would do the same as a mouse click.