From http://stackoverflow.com/questions/14854677/adding-fading-color-transparency-to-uiview

UIView Linear Gradient

//More Basic

    CAGradientLayer *gradient = [CAGradientLayer layer];
    gradient.frame = self.view.bounds;
    gradient.colors = @[(id)[[UIColor blackColor] CGColor],
                        (id)[[UIColor whiteColor] CGColor]];
    [self.view.layer insertSublayer:gradient atIndex:0];

//More Advanced
+ (void)addLinearGradientToView:(UIView *)theView withColor:(UIColor *)theColor transparentToOpaque:(BOOL)transparentToOpaque
{
    CAGradientLayer *gradient = [CAGradientLayer layer];

    //the gradient layer must be positioned at the origin of the view
    CGRect gradientFrame = theView.frame;
    gradientFrame.origin.x = 0;
    gradientFrame.origin.y = 0;
    gradient.frame = gradientFrame;

    //build the colors array for the gradient
    NSArray *colors = [NSArray arrayWithObjects:
                       (id)[theColor CGColor],
                       (id)[[theColor colorWithAlphaComponent:0.9f] CGColor],
                       (id)[[theColor colorWithAlphaComponent:0.6f] CGColor],
                       (id)[[theColor colorWithAlphaComponent:0.4f] CGColor],
                       (id)[[theColor colorWithAlphaComponent:0.3f] CGColor],
                       (id)[[theColor colorWithAlphaComponent:0.1f] CGColor],
                       (id)[[UIColor clearColor] CGColor],
                       nil];

    //reverse the color array if needed
    if(transparentToOpaque)
    {
       colors = [[colors reverseObjectEnumerator] allObjects];
    }

    //apply the colors and the gradient to the view
    gradient.colors = colors;

    [theView.layer insertSublayer:gradient atIndex:0];
}