Condtional formatting will do this for you.
If cell F2 contains the current spend and cell G2 contains the target spend or maximum:
Select cell F2
From the Menu select Format - Conditional Formatting ...
In the conditional formatting dialog box that opens, in the Condition 1 drop-down, select 'Formula Is'
In the box to its right enter this formula:
Click Format and in the Patterns Tab select a red color, click OK.
Back in the Conditional formatting dialog box click Add.
Under Condition 2 select 'Formula Is' and enter this formula:
Select a yellow color and add a third Condition with this formula:
and format the pattern Green,
F2 will have no background color if the cell is empty or contains zero.
It will be green if less than 90% of the value in G2
Then yellow for 90% to 100% of G2, and if greater it will be red.
If the relative positions of the Value (F2) and the Maximum (G2) are the same for other cells, you can copy and paste or drag to extend the formula and the conditional formatting will follow.
Note: In the formulas TRUE and FALSE do not have double quotes around them, they are Excel's logical states true and false.
Use Ctrl+V to paste the formulas into the formula boxes in the conditional formatting dialog boxes (there is no right-click menu - paste available). Thanks to DerbyDad03 for this tip.