I stared at this one for a long time: https://hidden-3d.com/index.php?id=gallery&pk=457 They use opposite colors for the foreground which makes it look opalescent and gray at the same time.
Ah the fading colours experiment! Interesting one!
I'm still not grasping the efforts it takes to create those effects..
I find this effect somewhat unpleasant to look at though.
I highly recommend having a look at some of his best creations on deviant art [0], particularly his technic of matching outlines with the final 3d result [1], or his experiments with transparency [2].
[0]: https://www.deviantart.com/3dimka [1]: https://www.deviantart.com/3dimka/art/You-must-not-fool-your... [2]: https://www.deviantart.com/3dimka/art/8-Legs-Magical-Stereog...