how to change font color on a [String] type?

I do not see where and how you change text in red.

In addition, please use code formatter (<>) to post code, and delete supefluous blank lines, much easier to read

class GameVC: UIViewController {
  
    @IBOutlet weak var storyLabel: UILabel!
  
    override func viewDidLoad() {
        super.viewDidLoad()
   
        if let Path = Bundle.main.url(forResource: "fortelling2", withExtension: "rtf") {
            do {
                let attributedStringWithRtf: NSAttributedString = try NSAttributedString(url: Path, options: [NSAttributedString.DocumentReadingOptionKey.documentType: NSAttributedString.DocumentType.rtf], documentAttributes: nil)
                self.storyLabel.attributedText = attributedStringWithRtf
                self.storyLabel.font = UIFont(name: "TimesNewRomanPS-BoldItalicMT", size: 30.0)
            } catch let error {
                print("Got an error \(error)")
            }
                   
            while  chosenWords.count > 1 {             
                let replace = storyLabel.text?.range(of: "§§§")
                 storyLabel.text?.replaceSubrange(replace!, with: chosenWords.removeFirst())
                }
            }
            }
  
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        /
    }
}

really sorry, here is the code i tried

import UIKit
import Foundation
class GameVC: UIViewController {
  
    @IBOutlet weak var storyLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
  
        if let Path = Bundle.main.url(forResource: "fortelling2", withExtension: "rtf") {
            do {
                let attributedStringWithRtf: NSAttributedString = try NSAttributedString(url: Path, options: [NSAttributedString.DocumentReadingOptionKey.documentType: NSAttributedString.DocumentType.rtf], documentAttributes: nil)
                self.storyLabel.attributedText = attributedStringWithRtf
                self.storyLabel.font = UIFont(name: "TimesNewRomanPS-BoldItalicMT",
                                              size: 30.0)
              
            } catch let error {
                print("Got an error \(error)")
            }
          
            let color : [UIColor] = [.red]
            let textRange = chosenWords
            let str = NSMutableAttributedString(string: textRange)
            for var i in 0..<textRange.characters.count {
                let range =  NSMakeRange(i, 1)
                let newColor = color[i % color.count]
              
                str.addAttribute(NSForegroundColorAttributeName, value: newColor, range: range)
            }
               
            while  chosenWords.count > 1 {
              
                let replace = storyLabel.text?.range(of: "§§§")
                storyLabel.text?.replaceSubrange(replace!, with: chosenWords.removeFirst())
                }  
            }
            }
  
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
       
    }
}

To change the text of the label, you need to set its attributedText property:

replace

storyLabel.text?.replaceSubrange(replace!, with: chosenWords.removeFirst())

with

storyLabel.attributedText?.replaceSubrange(replace!, with: chosenWords.removeFirst())

There are a few things I do not understand well in your code:

            let color : [UIColor] = [.red]
            let textRange = chosenWords
            let str = NSMutableAttributedString(string: textRange)
            for var i in 0..<textRange.characters.count {
                let range =  NSMakeRange(i, 1)
                let newColor = color[i % color.count]
      
                str.addAttribute(NSForegroundColorAttributeName, value: newColor, range: range)
            }

Line 2 : where is chosenWords defined ? How ? What is its value ?

line 4 : i is not modified, you could write for i in 0..<

line 5: color.count is 1 ; hence %1 will alwauys yeld 0 ; hence newColor = color[0] = .red, always

line 8 : which version of XCode ? In recent versions, key is foregroundColor , no more NSForegroundColorAttributeName