And on another tack .... there are web links on the subject of banding ... some deal with gradient banding (including colours).
One
here
Another one
here
And especially
Here
This last has this advice ....
'Fading a single color to transparency will not yield pretty results. If you absolutely must fade a color to transparency, make sure the gradient is small, or that the range of color difference in the gradient is high. It’s the long and subtle gradients that cause problems.' ..... so you could try fading the bad band to 'transparency'
'Use Noise in Another Layer. If you absolutely need to maintain a lot of integrity in your photo, but still need to remove banding in some area, try adding the noise to its own layer.' .... which is mentioned on other sites.
It goes on to add that to do this ....
1. Create a new layer and fill it with 50% grey. That’s about #808080 if you’re working in RGB.
2. Place this layer above the image you’re adding noise to. In the layers palette, choose ‘Overlay’ as the blending mode.
3. Add noise to this grey layer, 2-3% should suffice but you should look in the preview image before you commit anything.
and that you’ll notice that the noise does not affect any very bright or dark areas of your image. Because the noise is in ‘Overlay’ mode, you’re free to crank up the noise as high as needed, without destroying the quality of your highlights or shadows.
Another
benefit to having the noise in its own layer is that you can use your eraser tool to remove noise from areas where it is not needed. You could even take one step further, creating a complex mask and applying it to the noise layer, giving you full control over which areas are affected.
Finally: This
last website link seems to have cracked it ..... I won't try to explain as its illustrated.
Some food for thought ....