I have been restoring some old pictures using GIMP, and watching heaps of youtube videos. I purchased Topax DeNoise Al, which I found really great for removing noise. However, as you can see from the attached image, there is alot of discoloration in the sky. I have spent hours trying different things with Wavelet Decompose, tried selecting the area by colour, tried heal tool, clone tool, rubber tool, but I'm not having too much luck. I'm fairly new to this, and hope this question isn't too beginner for the forum. Thanks in advance, Lee.
You could use a soft edged paint brush, with the colour mode set to HSL Color in the Tool Options
Then use white as the foreground colour, or Ctrl+Click on an areas to select the colour you want, and paint over the areas which are discoloured. You might also want to reduce the Opacity of the brush so you can do it in a more controlled or gradual way.
Here's a very quick example. I'm pretty sure with a little more care and attention you could do better.
Another quicker, less manual approach, would be to remove the yellow colour-cast using a Hue-Saturation adjustment. Obviously this method might not work for other images where there are actual yellow objects. But with the example image it works quite well.
Here's a guess (no idea of the original image, only the attached one with very bad compression noise is available):
Apply Selective Blur. Here it's applied to the shown selection in the middle:
That blur makes the edges of the apparent dirt unsharp.
Different areas of the images need different settings, so make in different layers different versions and paint to the layer masks to make the wanted blurred versions visible in the wanted areas.