change the background color of cell B5 based on the value of another cell - C5. If C5 is greater than 80% then the background color is green but if it's below, it will be amber/red.
There is no mention that B5 contains any value so assuming 80%
is .8
formatted as percentage without decimals and blank counts as "below":
Select B5, colour "amber/red" with standard fill then Format - Conditional formatting..., Custom formula is and:
=C5>0.8
with green fill and Done.