There is a function known as Conditional formatting that allows you to set things such as cell or font color depending on the value in the cell itself or in another cell.
Before Excel 2007, you could set three cell formatting conditions, for example values less than 20 set cell green, 20 to 89 set cell yellow, 90 or over set cell red. In Excel 2007, a lot more choices exist.
For Excel 2003 do this:
Select cell to 'color'
From the menu select Format, Conditional formatting...
In the drop-down box select 'Cell Value Is'
From the next box select 'equal to' and in the last box enter 20,
finally click format and select the color / font /font color required.
Click OK and OK again.
If you want the color of the cell to change depending on a value in another cell select 'Formula Is' from the first drop-down and enter a formula in the next box, starting with equal.
e.g. =IF(A2=20, TRUE,FALSE)
then select the required format.
If the formula evaluates to TRUE, the selected format is displayed.
Note that TRUE and FALSE are not in quotes - they are Excel's logical values.
You can create quite complex formulas but the formulas must evaluate to either TRUE or FALSE.
Hope this helps.
If you are using Excel 2007, please ask in the Office Software Forum if you need further instructions as CF in Excel 2007 is more complicated.