Collisions work fine now, phew, that took me most of my afternoon (it’s now nearly 9 p.m here).

And the main gameplay mechanics work fine too
In the game, press Down to make Zip absorb the color of the tile below, changing his skin and allowing him to pass through obstacles of the same color !
The obstacle script will allow making minimalism-inspired level designs.

The example added doesn’t reflect quite well the possibilities of this gameplay, I’ll add a new level later, tonight hopefully.
Now remains the level structure (level objective, changing level obstacles), and minimalism-inspired special effects (win, lose, color switch).
If I can do this tonight, tomorrrow will be a pure level design day !

Here’s a link to Zip, playable in your favorite internet navigator:
http://www.teradem.net/HTML5/Zip/