I'VE GOT IT! grin
Thank you for your help, all of you!
I'll explain:
intially, i declared the bitmap BMAP* texture2 = "empty.bmp"
empty is a totally white 256x256 bitmap
the thing is, by setting the skin of 200 different entities to the same texture2, would, obviously make them all have the exact same skin
So i thought about this, and then i remembered bmap_createblack, which creates a black bitmap, with whatever size and format i want
eventhough i repetead the bmap declaration at the start of the script, i did something diferent:
instead of
BMAP* texture2 = "some image file";
i did this
BMAP* texture;
then, in the function that colours the bitmaps, i did some changes too

Code:
void bmap_create_draw(ENTITY* ent, var alpha)
{
	texture = bmap_createblack(256, 256, 888);
	
	var format = bmap_lock(texture, 0);
	var pixel;
	var width = bmap_width(texture);
	var height = bmap_height(texture);
	var i, j;
	
	for(j=0 ; j < width ; j++)
	{
		pixel = pixel_for_vec(vector(random(255),random(255),random(255)), alpha, format);
		for(i=0 ; i< width ; i++)
		{
			pixel = pixel_for_vec(vector(random(255),random(255),random(255)), alpha, format);
			pixel_to_bmap(texture, i, j, pixel);
		}
	}
	bmap_unlock(texture);
	
	ent_setskin(ent, texture, 1);
	
}


the algorithm's pretty much what it was initially, but now as an argument i use the pointer to the entity i want to have the skin being coloured, so that in the end i can do this
ent_setskin(ent, texture, 1);
and here's the main diference
texture = bmap_createblack(256, 256, 888);
the previously declared BMAP* texture; is now "generated" multiple times as a black bitmap, which is later coloured and skinned in one of the randomly generated and deformed asteroid entities
it works perfectly, now i have the basis to make these so called "asteroids" more realistic and generate them more dinamically