54F3

put the blue box in white and slide it to black so the entire box is a hole

But only if sliding it back to #FF0 returns the box to how it was. Otherwise black is kind of irreversible, and doing this is bad.

I'll get GIMP out and solve this this weekend.

You might want to reverse the quoting suggestions restriction when it comes to color changes.

Oh, it counted, but I thought you wanted to check before seconding it. It would be irreversible if you did that, for the record.