Oh I didn't read your what you said about the sponge before making my edit. I went for a blocky/geometric shape which I now find pretty damn ugly. The character designs I made were just for fun anyway.
Anyway, I'm not sure of what use this could be for you, the statement really was to try and make things simpler.
I'll agree that this is overly simple: the shading, and the perspective aren't every interesting but I think as a whole it's more coherent than what you have showed. The fact that I'm using 10 colours helps I guess, but I'm pretty sure you could get the same result with more colours.
I really tried to make the lines clean, since some of yours are a bit messy.
Also, I used the darkest colour of the table and sponge as the lightest colour of the background. I think this makes everything more coherent while still having the foreground flash like you wanted.
Bear with me, this has to be like my 5th attempt at pixel art.
(the wood texture lines weren't cleaned out because I'm lazy).
Je ne savais pas non plus que tu étais francophone avant de lire ton blog.