@Wilson L answer worked perfectly.
For those who code using Swift, here is the code:
func imageFromColor(color:UIColor) -> UIImage{
var rect:CGRect = CGRectMake(0, 0, 1, 1);
UIGraphicsBeginImageContext(rect.size);
var context:CGContextRef = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(context, color.CGColor );
CGContextFillRect(context, rect);
var image:UIImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}
backButton.setBackgroundImage(imageFromColor(UIColor(red: 8/255, green: 75/255.0, blue: 122/255.0, alpha: 1.0)), forState: .Normal)
backButton.setBackgroundImage(imageFromColor(UIColor(red: 255/255, green: 60/255.0, blue: 93.0/255.0, alpha: 1.0)), forState: .Selected)