NSString+HexStuff.m

@implementation NSString (HexStuff)

- (NSUInteger)hexValue
{
	NSUInteger i = 0;
	
	// -- strip leading #
	if( [self hasPrefix:@"#"] )
		i++;
	
	NSUInteger hex = 0x0;
	NSInteger x;
	NSString *s;
	for( ; i<[self length]; ++i )
	{
		s = [self substringWithRange:NSMakeRange( i, 1 )];
		x = [s integerValue];
		
		NSAssert( x < 0xa, @"Something is wrong, should always be less than 10" );
		
		if( x == 0 )
		{
			unichar c = [[s lowercaseString] characterAtIndex:0];
			switch (c)
			{
				case '0':
					x = 0;
					break;
				case 'a':
					x = 0xa;
					break;
				case 'b':
					x = 0xb;
					break;					
				case 'c':
					x = 0xc;
					break;
				case 'd':
					x = 0xd;
					break;
				case 'e':
					x = 0xe;
					break;
				case 'f':
					x = 0xf;
					break;
				default:
					return 0x0;
			}
		}
		hex += x << 4*([self length]-i-1);
	}
	return hex;
}

@end

UIColor+HexStuff.m

@implementation UIColor (HexStuff)

+ (UIColor *)colorWithHexString:(NSString *)string alpha:(CGFloat)alpha
{
	NSInteger startIndex = 0;
	CGFloat color[3] = { 0x0, 0x0, 0x0 };
	
	if( ([string length] == 7) && [string hasPrefix:@"#"] )
	{
		startIndex = 1;
	}
	
	if( ([string length]-startIndex) == 6 )
	{
		NSString *redString = [string substringWithRange:NSMakeRange(0+startIndex, 2)];
		NSString *greenString = [string substringWithRange:NSMakeRange(2+startIndex, 2)];
		NSString *blueString = [string substringWithRange:NSMakeRange(4+startIndex, 2)];
		
		color[0] = [redString hexValue];
		color[1] = [greenString hexValue];
		color[2] = [blueString hexValue];
	}
	return [UIColor colorWithRed:color[0]/0xff
						   green:color[1]/0xff
							blue:color[2]/0xff
						   alpha:alpha];
}
@end