[pdurrant]

Originally Posted by Mike L

For what it's worth:

Spoiler:

I don't understand why, but there are not as many colours as first appears. In fact, the blue and the green are apparantly the same colour.

To demonstrate, I copied the image into a graphics editor, and used the color picker tool to get the underlying colour values. For both the green and the blue, it showed up as RGB(0,255,150). That colour normally renders as a very bright green.

Somehow the interaction of the blue/green with the neighbouring colours is causing the effect. But, as I said, I don't really understand it.

Yes, well done. I suspect many people won't believe the answer until they do the same.