how to remove background-color from the row?

how to remove background-color from the row?

I have some data rows with alternate row color with following 2 CSS classes applied.
1. alt-1
2. alt-2
onMouseOver event of the row I replace the class "alt-2" and in onMouseOut event I replace the alt-1 back vice-verse.

and, when the user select the checkBox of the row. I need the background to change to different color. I used the background-color property of CSS directly and done it.

upto here works fine. My problem is, when user uncheck the check box I want to remove the background color from the selected row. How could I do that?