Use RC addressing. So, if I want the background color of Col B to depend upon the value in Col C and apply that from Rows 2 though 20:
Steps:
Select R2C2 to R20C2
Click on Conditional Formatting
Select "Use a formula to determine what cells to format"
Type in the formula: =RC[1] > 25
Create the formatting you want (i.e. background color "yellow")
Applies to: Make sure it says: =R2C2:R20C2
** Note that the "magic" takes place in step 4 ... using RC addressing to look at the value one column to the right of the cell being formatted. In this example, I am checking to see if the value of the cell one column to the right of the cell being formatting contains a value greater than 25 (note that you can put pretty much any formula here that returns a T/F value)