I've done a bit more testing and it definitely seems to be a rendering issue.
Attached is a simple test in ePub with various shades of grey. (It's deliberately a 2-page document to force a white 'cleaning' page before the actual tests.)
In calibre reader, ADE for PC and on my Sony PRS-300 all 8-shades show up differently, (although the dithering is quite apparent on the Sony with some of them - presumably because it only has 8 actual greys to use).
On Kindle PW the results are very different:
White and Grey 1 are both rendered as white.
Greys 2 to 5 are light grey but all the same.
Greys 6 and 7 are the same as each other but darker than 2 to 5.
If anyone feels inclined to test on their own reader and report back I would appreciate it. (I want to make sure my books render as expected on any device).
Last edited by Agama; 01-11-2013 at 06:27 PM.