It can be done fairly easily with vba my looking at each cell information in a range and then just adding up, but i a fairly sure there is no straightforward way with a formula.

How were you planning on colouring the cells? Manually or with Conditional formatting?

If you are going to have conditional formatting it will need to have a rule (minus sign or sell etc) and if that is the case this is a much more reliable way of counting

If manual we need to ask the question of 'what is green'? is it the dark green or olive green or another shade

Adding a workbook is always a help