[GeoffC]

Originally Posted by pdurrant

No, that's not it. I've added another picture with the centre removed (because I've also turned off anti-aliasing with the new picture, and the centre detail is too fine to work without anti-aliasing).

If there is any apparent warming of the colours towards the centre, it's an artefact of the conversion from an eps to pixels.

The illusion isn't a small effect that you have to strain to see.

Spoiler:

It doesn't matter what you've done, even with the centre detail removed there is still an apparent change in the colour from the outer to the inner.

No straining involved.....