The strange variety of background colors can add to the charm and mystery of these DC books, but often do add a bit of "Lucy In The Sky" surrealism. I agree that the Adventure #284 would have looked nicer with blue sky and a few clouds and that it would have not taken much extra effort. In fact, I shall re-color mine!
Concerning scans, I always do two. I scan for myself at 200dpi and keep these on a separate hard drive. for the boards I scan at 96dpi. The colors of both are fairly consistent, but the higher dpi scans are much purer when compared with the 96dpi, the latter ones often looking less high grade than they really are.
I agree your colors look good on your scans, but I would like to see some of your 200 dpi scans, as I think that is why I thought they looked "grainy". Mine are done at 200 and I don't think they are any bigger than yours in size. Your scans are a bit bigger than mine.
I am working on a project to scan every silver/bronze book I have now, starting with the 10 centers....I hope you guys aren't tired of these books yet as I have alot more to scan
Instead of using a hard drive to store my scans I upload them to Photobucket. It will be some time before I run out of 5 GB of storage for comic covers..