Numbers 6.2.1 -- Conditional Cell Colours -- Positive Green, Negative Red

I just made a spreadsheet and have one important metric. I want to highlight the cells to show if I'm "ahead" or "behind." If the number is positive I'd like the cell to show green and if the value is negative a red cell.

I assume the answer has something to do with Format / Conditional Highlighting but I've never done anything like this and am not sure how to proceed.

Any advice would be much appreciated.

Thanks