Formatting list boxes from script

How would I go about formatting a list box based off of the output of another list box?

I am trying to change the background color of a list box = "B" based on the output of another list box "A". list box "A" has two options the first being 1 and the second being 2. I want the background of list box "B" to change to the color yellow when option 1 in list box "A" is chosen and stay white when the option 2 is chosen.