Formula needed for conditional formatting

I need a formula that will look at column B, recognize an empty cell in B and color the corresponding cell in column A a different color.

So: Column A Column B
1 02/12/13 02/20/13

2 02/10/13

3 01/16/13 01/15/13

Column B2 would trigger the formula to turn A2 red or blue or whatever color I set. It would need to cover a date range such as 06/01/12 to NOW().