Color parts of words in attributedString. Note: Also respects any previous attributes set in Storyboard (colored words or lineHeightMultiple etc.). Also a simple function which just makes a new NSMutableString with the defined text value while keeping all existing attributes.

NSAttributedString+MakeMutable.swift

extension NSAttributedString {
    
    // MARK: - Makes a copy of the existing attributes for NSAttributedString and sets color for specific string(s)
    func makeStringWithColor(forString stringArray: [String], withColor color: UIColor) -> NSMutableAttributedString {
        
        // Existing attributes
        let attributedString = NSMutableAttributedString(attributedString: self)

        // Iterate through strings
        for specifiedString in stringArray {
            
            // Whole range (for regex)
            let range = NSMakeRange(0, self.string.characters.count)
            
            // Find several occurences
            do {
                let regex = try NSRegularExpression(pattern: specifiedString, options: .caseInsensitive)
                
                // Enumerate matches
                regex.enumerateMatches(in: self.string, options: .reportCompletion, range: range, using: { (result, _, _) in
                    
                    // Substring range
                    if let subStringRange = result?.rangeAt(0) {
                        attributedString.addAttribute(NSForegroundColorAttributeName, value: color, range: subStringRange)
                    }
                })
            }
            
            catch {
                print(error)
            }
        }
        
        return attributedString
    }
    
    // MARK: - Makes a copy of existing attributes for NSAttributedString and returns a new NSMutableString with a new text / string.
    func makeStringWith(text newString: String) -> NSMutableAttributedString {
        
        // Keep existing attributes
        let attributedString = NSMutableAttributedString(attributedString: self)
        
        // Update string, keep attributes
        attributedString.mutableString.setString(newString)
        
        return attributedString
    }
}