Keep the colors of the textView text after appending NSSting

I'm using the next method to change the colors of few words in textView:

+(void)changeColorToTextObject : (UITextView *)textView ToColor : (UIColor *)color FromLocation : (int)location length : (int)length
{
    NSMutableAttributedString *text = [[NSMutableAttributedString alloc] initWithAttributedString: textView.attributedText];

    [text addAttribute:NSForegroundColorAttributeName value:color range:NSMakeRange(location, length)];
    [textView setAttributedText: text];
}

and it looks like this:

but when i'm adding new value to this text the color are gone.

myTextView.text = [myTextView.text stringByAppendingString:newStr];

And it looks like this:

How can I keep the colors like before with the new string?