Swift attributed string part colored issue

Question

I'm trying to change the second half of some text to a different color using range.

        let stringToColor = NSLocalizedString("Plan", comment: "")
        let mainString = title + ": "  + stringToColor
        let range = (mainString as NSString).range(of: stringToColor)
        let attrString = NSMutableAttributedString(string: mainString)
        attrString.addAttribute(NSAttributedStringKey.foregroundColor, value: UIColor.orange, range: range)

The text gets outputted but all one color, not the one set using foregroundColor key. Just can't see what the issue is.

ty