gistfile1.m

- (UIColor *)colorByChangingAlphaTo:(CGFloat)alpha
{
    CGFloat *oldComponents  = (CGFloat *)CGColorGetComponents([self CGColor]);
	size_t numComponents    = CGColorGetNumberOfComponents([self CGColor]);
    
	CGFloat newComponents[4];
    
	switch (numComponents)
	{
		case 2:
		{
			//grayscale
			newComponents[0] = oldComponents[0];
			newComponents[1] = oldComponents[0];
			newComponents[2] = oldComponents[0];
			newComponents[3] = alpha;
			break;
		}
		case 4:
		{
			//RGBA
			newComponents[0] = oldComponents[0];
			newComponents[1] = oldComponents[1];
			newComponents[2] = oldComponents[2];
			newComponents[3] = alpha;
			break;
		}
	}
    
	CGColorSpaceRef colorSpace  = CGColorSpaceCreateDeviceRGB();
	CGColorRef newColor         = CGColorCreate(colorSpace, newComponents);
	CGColorSpaceRelease(colorSpace);
    
	UIColor *color = [UIColor colorWithCGColor:newColor];
	CGColorRelease(newColor);
    
	return color;
}

- (UIColor *)invertColor
{
    CGColorRef oldCGColor   = self.CGColor;
    size_t numberOfComponents  = CGColorGetNumberOfComponents(oldCGColor);
    
    if (numberOfComponents == 1)
        return [UIColor colorWithCGColor:oldCGColor];
    
    const CGFloat *oldComponentColors = CGColorGetComponents(oldCGColor);
    CGFloat newComponentColors[numberOfComponents];
    
    NSInteger i             = numberOfComponents - 1;
    newComponentColors[i]   = oldComponentColors[i];
    
    while (--i >= 0)
        newComponentColors[i] = 1 - oldComponentColors[i];
    
    CGColorRef newCGColor   = CGColorCreate(CGColorGetColorSpace(oldCGColor), newComponentColors);
    UIColor *newColor       = [UIColor colorWithCGColor:newCGColor];
    CGColorRelease(newCGColor);
    
    return newColor;    
}