OK, thanks for the link, I thought plugins were stored in the scirra store and found nothing
I read the plugin description: this doesn't really answer to my questions...
Last night I managed to create more or less what I wanted: random generation of mountains, suns, with a background color that changes (as I described in my first post), can swap between foreground and background.
The only thing that is missing is this: when two black sprites are overlapping, I want the overlap to be white (black is the color I then replace with any color I want).
I thought I could do that by putting sprite 1 on a layer and Sprite 2 on another layer, and use the "exclusion" or "difference" effect, but it won't work (or I'm doing it wrong).
That's really the last thing that's missing, and I feel installing the Canvas plugin won't necessarily solve this problem (and will make a lot of what I made until now obsolete).
I'll post the .capx when I get back home tonight to show you what I mean.