For a jump n' run or something like that I advice you to make
a layer (which is invisible in your game) with spezial colors
and then you can use the pixelfunctions to simulate collisions.
That's the way it's discribed in the AUM.

Then you can use different colors for Whater etc. also, and thats very useful,
just a bit slow, and if you want to make collisions with enemies you have to
look for another solution.
(just painting with pixel_to_bitmap on the collisions-bitmap over the enemies is too slow)