There are several ways to go about it depending on your code
and preferences.
Assuming you have a background in the Screen already (Sprite
or MovieClip or other Display Object):
1. You might want to change its color using ColorTransform
class.
2. You can replace existing background with a new one (say,
Sprite) and draw a rectangle of a desired fill color inside it
using yourSprite .graphics class. In this case place a new
background into the same stack order as your previous one - old one
will be automatically replaced. Don't forget to mark the previous
one for garbage collection.
3. You can draw a new rectangle inside the existing
background, again, using drawing API (graphics class). Don't forget
to invoke clear() method to delete previously drawn rectangle
before you draw a new one.
Hope it helps.