Why the screen of Canvas does not change color when I use UIColor in SwiftUi or UIKit

Using Swift
, ,
1

When I run ContentView.swift code as follows

import SwiftUI
import UIKit


class MainViewController: UIViewController{
    
  
    
    let collectionView: UICollectionView = {
        let layout = UICollectionViewFlowLayout()
        let cv = UICollectionView(frame: .zero, collectionViewLayout: layout)
        let darkGrey = UIColor(hexString: "#757575")
        cv.backgroundColor = UIColor(hexString: "#757575")
        return cv
    }()
    
    override func viewDidLoad(){
        super.viewDidLoad()
        
        view.addSubview(collectionView)
        collectionView.frame = view.frame
        
    }
    
    
}

extension UIColor {
    convenience init(hexString: String) {
        let hex = hexString.trimmingCharacters(in: CharacterSet.alphanumerics.inverted)
        var int = UInt64()
        Scanner(string: hex).scanHexInt64(&int)
        let a, r, g, b: UInt64
        switch hex.count {
        case 3: // RGB (12-bit)
            (a, r, g, b) = (255, (int >> 8) * 17, (int >> 4 & 0xF) * 17, (int & 0xF) * 17)
        case 6: // RGB (24-bit)
            (a, r, g, b) = (255, int >> 16, int >> 8 & 0xFF, int & 0xFF)
        case 8: // ARGB (32-bit)
            (a, r, g, b) = (int >> 24, int >> 16 & 0xFF, int >> 8 & 0xFF, int & 0xFF)
        default:
            (a, r, g, b) = (255, 0, 0, 0)
        }
        self.init(red: CGFloat(r) / 255, green: CGFloat(g) / 255, blue: CGFloat(b) / 255, alpha: CGFloat(a) / 255)
    }
}

I get a white screen on Canvas and I don`t understand why my code does not work since I built an extesion UIColor. I tried also writing cv.backgroundColor = .green without extesion UIColor but I obtain the same result. Any idea would be appreciated! Thank you for your patience.