Did not manage to do it that way.

I ended up calling the blank cells "z" (say) and then by colouring the cells
green to begin with, I used the 3 conditions to
1.) colour any z's with a white background and white font
2.) highlight in amber -1's and 0's
3.) highlight in red values less than -1.

Messy but it finally worked.

Thanks for the other tips, will come in useful.