A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Let's say the forecast values are in C2 and down, and the actual values in D2 and down.
Select the range with actual values; I will assume that D2 is the active cell in the selection.
On the Home tab of the ribbon, click Conditional Formatting > Highlight Cells Rules > Greater Than...
Enter =10%*C2 in the box, then click OK or press Enter.
If you actually meant to highlight actual values that are at least 10% higher than the forecast values, use =110%*C2 instead.