How to remove a line from NULL_BRUSH background

[Sascha W.]

I want to rotate a single line on a "empty" background. I used the NULL_BRUSH option in the GetStockObject, it works, but if I draw a new line on an other position the old one appeared too! So how is it possible to remove the old one?
By the way, how to set a new color to the new line?
Thanks and regards
Sascha