If tick boxes are ticked - range of cells changes fill colour

Basic example of what I need is:

Say, cell range a1:e10 are currently white fill. there are two tick boxes... when I tick one of them, I want nothing to happen to the cells, but when BOTH are ticked, I want the cells to change colour .e.g grey them out because they are 'done' .

how do I do this.

thanks

oh yeah, didn't even think of that easy option lol thanks. ive even got that exact thing going on, in another spreadsheet ha