Colour fomatting a cell

In my spreadsheet I would like (for example) cell AU6 to turn red if the value in cell AU is 0. Alternatively it can turn red if the cell value of cell AP6 is 0. I've tried all variations in conditional formatting and cannot get it to work. Would appreciate help and am attaching the spreadsheet.

If you put a number in AP - say 1, and a number in AQ - say 4 then AU will be 25. It will turn red.

However if you type 0 in AP and 4 in AQ, then AU will be 0. I can't get AU to turn red with conditional formatting. Frustrating!

The problem seems to be that when you type 4 in AP and 4 in AQ, AU becomes 100 and because it has a 0 in it, the cell turns red when it needs to turn green. So any formula that says if it has a 0 in it doesn't seem to work.

AS and AT work as they never have a 0 in them. AS turns green with a certain percent (though I have not used percentage - just the number over 100 as you can see in the formula) and AT turns yellow - Perhaps I've made work for myself and one column could have been used instead of 3. In that column I could have a return of 0 - 78 turns red, 78 - 89 turns yellow and 90 - 100 turns green. Wonder if that would be better. Will have a play and come back. Perhaps it will solve the problem. I tried your formula in conditional formatting and it didn't work for me. I'm presuming it worked for you!

Sometimes just talking to someone triggers the answer and I thank you for letting me talk - doing it in one column instead of 3 worked a treat! Thanks

Sorted it now - thanks for the effort you've put in. It's working just fine now.