Thank you, Nick, for your prompt and helpful response. I would have got back to you sooner, but I was jiggling with all the settings in order to see if I came across any other problems or questions.
In short, I incorporated your suggestions by increasing colours to 256, with "no edge enhance", dpi 600, plus using the "pdfread-MinFilter5-mod-bin.zip " executable, and "no dilation".
The result was that the shading was only very faintly visible to the naked eye on both Mobipocket and Calibre Readers, barely visible on a screenshot and, more importantly, not noticeable on my Cybook.
So that's a practicable solution.
As a separate issue, I had to use "landscape-half" to make the fonts readable on the Cybook; am I correct in assuming that this will vary from book to book? And is there any way of over-riding the preset categories in "Layout Mode" to provide more precise gradations from book to book?
One final query: I specified the "Size" as "H480V620"; is this the correct way to do it for the Cybook?
And one final comment; I can't help feeling that all the problems may stem from the GUI's miss-spelling of the words "colour" (TWICE!) and "grey".
