I don't know if maybe it's a difference in Excel version but this question is 6 years old and the accepted answer didn't help me so this is what I figured out:
Under Conditional Formatting > Manage Rules:
- Make a new rule with "Use a formula to determine which cells to format"
- Make your rule, but put a dollar sign only in front of the letter:
$A2<$B2
- Under "Applies to", Manually select the second column (It would not work for me if I changed the value in the box, it just kept snapping back to what was already there), so it looks like
$B$2:$B$100
(assuming you have 100 rows)
This worked for me in Excel 2016.