Shows how to apply alpha masked tintColor to a UIImage in Swift 4.

UIImage+TintColor.swift

import UIKit

extension UIImage {
  func tinted(with tintColor: UIColor) -> UIImage {
    guard let cgImage = cgImage else { return self }
    
    UIGraphicsBeginImageContextWithOptions(size, false, scale)
    defer { UIGraphicsEndImageContext() }
    
    guard let context = UIGraphicsGetCurrentContext() else { return self }
    
    context.translateBy(x: 0, y: size.height)
    context.scaleBy(x: 1, y: -1)
    context.setBlendMode(.normal)
    
    let rect = CGRect(origin: .zero, size: size)
    context.clip(to: rect, mask: cgImage)
    
    tintColor.setFill()
    context.fill(rect)
    
    guard var tintedImage = UIGraphicsGetImageFromCurrentImageContext() else { return self }
    
    if capInsets != .zero {
      tintedImage = tintedImage.resizableImage(withCapInsets: capInsets, resizingMode: resizingMode)
    }
    
    return tintedImage
  }
}