Changing the windows backcolor at runtime

Hello,
how can I change the backcolor of my window at runtime? Actually, I created three scrollbar to change the backcolor in RGB. When the scrollbar change, it get it's value stored in a variable name red, green and blue. When the user click OK, my backcolor would change...

I looked into MSDN, and SetWindowLong function can't be use to change the backcolor. It can be use to change Extended Styles, Styles, Window Proc, ID, Instance and User Data.