You can use conditional formatting to do this. Highlight all the cells to be affected, then click Conditional Formatting | New Rule | Use a Formula ..., then put this formula in the box:

=$B2="L"

(NOTE: this assumes that row 2 is the first highlighted row - change the 2 if different)

Then click on the Format button, choose the Color drop-down and click on Red. Then you can OK your way out and Excel will automatically adjust the formulae to suit the cells you had highlighted.

Hope this helps.

Pete