Update: I made a few minor changes to the clouds (accentuated a few curls) and implemented the code - actual screenshot:
(The jerks are due to the time that passes between the instructions
poke53272,gp : poke53265,91, where the first changes the video memory location and the second the video mode. I've already managed to hide them by making them occur during the horizontal blanking, but the solution isn't 100% stable, so there's still work to do. EDIT: fixed, now.)
I've also been thinking about highlights...
The first solution that came to mind was to turn the dents into bumps and thus use the 3 background colors for both them and the highlights; yet, this idea didn't convince me at all.
The second solution was to just turn the dents by 180° so that they'd show the bright side, which could be painted in the highlight colors: however, a quick mockup (which I trashed, sorry) didn't manage to convince me.
The third solution came from an old idea that actually got implemented in the first stages of development:
It surely makes things more interesting, but for some reason the slanted top side doesn't really seem suitable to me (and it also costs the loss of gray as shadow color). What do you say?
EDIT: alternative (and better) highlights:
#1 requires only 1 character, the others need 3 characters and more processing time (but those are hardly problems). I can't really decide, probably the best - as it often happens - is in the middle
Also, I've been considering (not for the first time) to remove the outlines, but they are too important both for the cartoonish feel and the clear separation of the platforms.
A final thing to note is that I have now 13 characters free (courtesy of the bitmap top): maybe they could be used to improve the platforms (f.ex., they could be used to add dents also to the slanted sides)...
EDIT 2:
Just tried a different approach:
The problem is that I can't figure out how to enrich the platforms with the 4 colors used for highlights and outlines (white, the two lighter grays and black)
EDIT 3:
Some more ideas:
Only the first looks acceptable to me, but I don't know if I'll go for it as the highlights make the platforms seem too smooth (and there's also the loss of gray for dents that bothers me). Ah, please ignore the bottom-right one: it isn't tecnically possible (that's what happens when one doesn't sleep enough)