从16进制字符串生成颜色

+ (UIColor *)colorFromHex:(NSString *)hexString alpha:(CGFloat)alpha
{
    int i=0,temp=0,n;
    const char *s = [hexString cStringUsingEncoding:NSUTF8StringEncoding];
    if (hexString.length < 6) {
        return nil;
    }
    int colorC[3];
    for (int index = 0; index < 3; index ++ ) {
        for(i = 0; i < 2; i ++ )
        {
            char a = s[i + index * 2];
            if(a>='A' && a<='F')
                n = a - 'A' + 10;
            else if(a>='a' && a<='f')
                n = a - 'a' + 10;
            else n = a - '0';
            temp= temp * 16 + n;
        }
        colorC[index] = temp;
        temp = 0;
    }
    
    UIColor *color = [UIColor colorWithRed:colorC[0]/255.0 green:colorC[2]/255.0 blue:colorC[2]/255.0 alpha:alpha];
    return color;
}