Use conditional formatting under the format menu. You would use if "cell value is equal to X" then select red under options.
Select the range of interest.
Activate Format|Conditional Formatting.
Choose "Formula Is" for Condition 1.
Enter as formula:
=AND(ISNUMBER(SEARCH("X",A1)),NOT(ISNUMBER(SEARCH("+",A1))))
Activate Format.
Choose red on the Patterns tab.
Click OK.
Activate Add.
Choose "Formula Is" for Condition 2.
Enter as formula:
=AND(ISNUMBER(SEARCH("+",A1)),NOT(ISNUMBER(SEARCH("X",A1))))
Activate Format.
Choose yellow on the Patterns tab.
Click OK, OK.
If you rather have an upper case X as condition, replace SEARCH by FIND in the above formulas.
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In addition to Skrappy's idea, you can try this approach. One advantage is that you are not limited to 3 conditions, in case you want to have more than 3 characters determine the cell color.
Right click on your sheet tab, left click on View Code, and paste this in.
This code is in a Case structure, so you can easily modify it or add to it if you need to in the future.
'''''''''''''''''''''''''''''''''''''''''
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
Select Case Target
Case "X"
Target.Interior.ColorIndex = 3
Case "+"
Target.Interior.ColorIndex = 6
Case Else
Target.Interior.ColorIndex = 0
End Select
End Sub
'''''''''''''''''''''''''''''''''''''
HTH
Tom Urtis