Equivalent color w/ alpha for UINavigationBar's barTintColor

UIColor+iOS7NavigationBar_Expanded.m

@implementation UIColor (iOS7NavigationBar_Expanded)
// Returns equivelent color with alpha nearest to minimum alpha
- (UIColor *) equivalentNonOpaqueColorWhenInterpolatedWithBackgroundColor: (UIColor *)backgroundColor minimumAlpha: (CGFloat) alpha
{
    NSAssert(self.canProvideRGBComponents, @"Self must be a RGB color to use arithmatic operations");
    NSAssert(backgroundColor.canProvideRGBComponents, @"Self must be a RGB color to use arithmatic operations");
    NSAssert(self.alpha == 1, @"Self must be an opaque RGB color");
    NSAssert(backgroundColor.alpha == 1, @"Background color must be an opaque RGB color");
    
    CGFloat r, g, b, a;
	if (CGColorGetNumberOfComponents(self.CGColor) == 2)
	{
		CGFloat w;
		if (![self getWhite:&w alpha:&a]) return nil;
		r = g = b = w;
	}
	else if (![self getRed:&r green:&g blue:&b alpha:&a]) return nil;
    
    CGFloat r2,g2,b2,a2;
	if (CGColorGetNumberOfComponents(backgroundColor.CGColor) == 2)
	{
		CGFloat w2;
		if (![backgroundColor getWhite:&w2 alpha:&a2]) return nil;
		r2 = g2 = b2 = w2;
	}
	else if (![backgroundColor getRed:&r2 green:&g2 blue:&b2 alpha:&a2]) return nil;
    
    
    CGFloat red,green,blue;
    
    alpha -= 0.01;
    do
	{
        alpha += 0.01;
        red = (r - r2 + r2 * alpha) / alpha;
        green = (g - g2 + g2 * alpha) / alpha;
        blue = (b - b2 + b2 * alpha) / alpha;
    }
	while (alpha < 1 && (red < 0 || green < 0 || blue < 0 || red > 1 || green > 1 || blue > 1));
    
    UIColor *new = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
    return new;
}
@end