The easiest way is to use Conditional Formatting
Conditional Formatting
http://www.mvps.org/dmcritchie/excel/condfmt.htm
The formula used in Conditional Formatting is a worksheet formula
and you want to think in terms of selecting an entire column rather
than an individual cell at a time.
select column C1 then column C, just so you have C1 as the active cell
format, conditional formatting
formula is: =C1>20 is going to be True (used) or False (ignored) errors are ignored
format button, use patterns then chose color for interior