title: GPUImage masking transparency
body: <p>I'm using GPUImage framework of Brad Larson in my app and I need to make a mask of mask.jpg and photo. So mask images are two colors images with black background and white geometric figure. So basically masking is working, the problem is that I can't make the transparency of mask. To make that I need to replace the black color of mask.png to lighter color (more gray to white). <br><br>
  So I tried to use code from here <br>
  <a href="https://stackoverflow.com/questions/7171679/want-to-change-a-particular-color-inside-an-image-with-another-color-iphone">Want to change a particular color inside an image with another color - iPhone</a> <br>
  But it doesn't work, the alpha is set to 0 and I don't know why. I tried to debug and everything seems to be fine, but it returns an UIImage with transparent color instead of black color.
  Is there a way to make it without coregraphics. Maybe coreimage?</p>