script for calculated-read only field that will change color based on condition

I need help creating a script that would make a calculated-read only field change color if a condition is met.  Should the script placed in the calculated event?

The fields are as follows:

CourseNum (auto sum calculates 11 numeric different fields)

A1P (numeric calculated read only field)

AP (numeric calculated read only field)

HIGHA (the sum of A1P and AP) - this is also a calculated read-only field

So the condition would be:

if CourseNum >20 and HIGHA >=50%

then change the background color of HIGHA to yellow

otherwise it should be white