Hi Ian,

Welcome to the forum.

Open the conditional formatting and have below three approaches there :-

>=0
====================
Formula =a1>=0%
and format the cell pattern as green


within 10% of the next cell
====================
=And(a1>0%,a1<=b1*10%)
and format the cell pattern as Amber


greater than 10%
=====================
=a1>b1*10%
and format the cell pattern as Red

where A1 is the cell on which CF logics are getting applied and B1 is the other cell

Above are just the idea to help you proceed further as I guess they won't work if you use them directly in Excel.
Any issues, post the sample file. Thanks.

Regards,
DILIPandey

<click on below 'star' if this helps>