Changing a formula in a cell by entering a number in another cell, is it possible?

Thanks again, appreciate your time.

The idea is if i enter 43 (for example) in to B20, then B22 would turn Green within 5 either side of 43 (range 38 to 48)
It would turn Amber within 13 either side of 43 (range 30 to 37 and 49 to 56)

What I'm hoping is to enter any value in to B20 and it would follow these sets of rules in B22

Sorry if I'm not being very clear.