Tinting UIImage with Overlay (preserving grey scale info)

UIImageTint.swift

import UIKit

extension UIImage {
  
  func imageGreyScale() -> UIImage {
    let imageRect = CGRectMake(0, 0, self.size.width, self.size.height)
    let greyContext = CGBitmapContextCreate(
      nil, UInt(self.size.width*self.scale), UInt(self.size.height*self.scale),
      8, 0,
      CGColorSpaceCreateDeviceGray(),
      CGBitmapInfo.fromRaw(CGImageAlphaInfo.None.toRaw())!
    )
    let alphaContext = CGBitmapContextCreate(
      nil, UInt(self.size.width*self.scale), UInt(self.size.height*self.scale),
      8, 0,
      nil,
      CGBitmapInfo.fromRaw(CGImageAlphaInfo.Only.toRaw())!
    )
    CGContextScaleCTM(greyContext, self.scale, self.scale)
    CGContextScaleCTM(alphaContext, self.scale, self.scale)
    
    CGContextDrawImage(greyContext, imageRect, self.CGImage)
    CGContextDrawImage(alphaContext, imageRect, self.CGImage)
    
    let greyImage = CGBitmapContextCreateImage(greyContext)
    let maskImage = CGBitmapContextCreateImage(alphaContext)
    let combinedImage = CGImageCreateWithMask(greyImage, maskImage)
    return UIImage(CGImage: combinedImage)
  }
  
  func imageWithOverlayTint(tintColor: UIColor) -> UIImage {
    let greyImage = self.imageGreyScale()
    UIGraphicsBeginImageContextWithOptions(self.size, false, 0)
    let bounds = CGRectMake(0, 0, self.size.width, self.size.height)
    greyImage.drawInRect(bounds)
    tintColor.setFill()
    UIRectFillUsingBlendMode(bounds, kCGBlendModeOverlay)
    greyImage.drawInRect(bounds, blendMode: kCGBlendModeDestinationIn, alpha: 1.0)
    let tintedImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return tintedImage
  }
}