Thank you Makslane for your rapid answer.
Well, I think I have found another way
On my project (successive pictures), my goal is to mark some areas of my pictures so that the starship explodes when it enters them. The problem is that these areas are much more complex than a simple square (so that testing x,y coordinates is not a precise enough solution). So, under a graphical tool, I open these pictures and put black color on these areas (then load these new "simplified" pictures under GE and put them under the original ones, using Z-depth): that's why I wanted to test colors during the game.
But there is a much more simple way to do that, using the collide event. Collision areas remain in black, whereas other areas are of transparent color. Because the GE collide function do not apply on transparent areas, my problem is solved and the solution works
Here again, GE is a great tool which increase productivity!