如何在UIImage上制作一种透明的颜色？

import UIKit

extension UIImage {
    func imageWithTransparentColor(opacity: CGFloat) -> UIImage? {
        let rect = CGRect(origin: .zero, size: size)
        UIGraphicsBeginImageContextWithOptions(rect.size, false, scale)
        let context = UIGraphicsGetCurrentContext()
        context?.setBlendMode(.multiply)
        let transparentColor = UIColor(white: 0, alpha: opacity)
        context?.setFillColor(transparentColor.cgColor)
        context?.fill(rect)
        draw(in: rect, blendMode: .multiply, alpha: 1)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage
    }
}

let image = UIImage(named: "myImage.png")
let transparentImage = image?.imageWithTransparentColor(opacity: 0.5)