UIColorから16進数の文字列を得る
NSString*colorHex = [self getColorHexByUIColor:[UIColor redColor]];
NSLog("%@",colorHex);

-(NSString*)getColorHexByUIColor:(UIColor*)color
{
    CGFloat r,g,b,a;
    [color getRed:&r green:&g blue:&b alpha:&a];
    return [NSString stringWithFormat:@"%02x%02x%02x",(int)(r*255.0),(int)(g*255.0),(int)(b*255.0)];
}