Helen,

This is feature of Excel, known as conditional formatting. I wasn't aware
that the feature imported into PPT.

Within Excel, <format menu, conditional formatting>.  
The conditional formatting dialog box, allows you to set up to three
conditional statements for each cell or cell range. The conditional statement
must evaluate to a true or false answer.

There are a series of pre-defined conditions:
    cell value is , <between> value 1 and value 2
    cell value is , <equal to, etc.> value 1
    cell value is , <etc.>

or you can use create a formula as the conditional statement.
Formulas allow you to test for multiple and simultaneous conditions.

Although, the conditional formatting dialog box tells you that you are
testing for only three conditions, you are actually testing for four. The
fourth state is the neutral state, or when all three conditions are false.

If condition one is true, then you can change the the cell's Pattern(color
or pattern fill) , font  or border characteristics.