Help- conditional cell formatting

Hello,

I am trying to do some conditional formatting and having a little trouble. Here is what Im looking to get.

I have a column A (estimated price) and Column B (purchased price). I would like to have column B's cell color to change to red if its greater than the value in the adjacent column A cell, and turn to green if that less than.

I can make it work on only one set of cells (lets say B2 compared to A2) but i do not know how to continue this formula for the entire column. I can only do the formating in compare to a single cell (say A2) instead of always reading the corresponding adjacent cell.

Thanks!