A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
Let's say the data are in A2:B100 (with headers in A1:B1).
Select this range. The active cell should be in row 2, i.e. either A2 or B2.
On the Home tab, select Conditional Formatting > New Rule...
Select 'Use a formula to determine which cells to format'.
Enter the formula
=$A2<>$B2
Note the $ signs before the column letters. 2 is the row number of the active cell in the top row of the selection.
Click Format...
Activate the Fill tab.
Select yellow.
Click OK, then click OK again.