title: Replace a color/colour in a SKSpriteNode
body: <p>I want to change a single, specified colour in my SpriteKit sprite to alpha, leaving all other colours unaffected. I don't think that there is an 'easy' way to do this and that a custom shader might be required.</p>
  
  <p>Does anyone know how to do this in Swift or Objective-C?</p>
  
  <p>Here's the image - I design it with a black background as it's easier to see but in my program, I want the background to be transparent.</p>
  
  <p><a href="https://i.stack.imgur.com/B2CkX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/B2CkX.png" alt="enter image description here"></a></p>
  
  <p>If I ry to edit the image with the background already set as alpha, then the package I'm using (Pixaki) show this as white-and-grey checks, so it looks like this:</p>
  
  <p><a href="https://i.stack.imgur.com/J4vfV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/J4vfV.jpg" alt="enter image description here"></a></p>