change the uiview background color with code

You can set the backgroundColor property of whatever view it is you have on screen.

In Objective-C:

self.view.backgroundColor = [UIColor redColor];

In Swift:

self.view.backgroundColor = .red

or if it's the main window you're after,

In Objective-C:

self.window.backgroundColor = [UIColor redColor];

In Swift:

self.window.backgroundColor = .red

self.view.backgroundColor = [UIColor redColor];

possible colors are :

blackColor  
 darkGrayColor  
 lightGrayColor  
 whiteColor  


 grayColor  
 redColor  
 greenColor  
 blueColor  
 cyanColor  
 yellowColor  
 magentaColor  
 orangeColor  
 purpleColor  
 brownColor  
 clearColor

For Swift 3, you should do:

self.view.backgroundColor = UIColor.white

Unfortunately, the other answers no longer work in Swift 3.

If you want to change the background color of the view with code in Swift, you should do instead:

self.view.backgroundColor = UIColor.redColor();