There are 3 blocks:
COSTUME: Return the x costume of the current sprite
NEW COSTUME COLOR RGB W x H: Return a costume of size width x height with uniform color RGB
COMBINE PIXELS FROM COSTUME TO COSTUME: Add pixels of costume1 into costume2 and return the resultas a costume.
Example: You can make a 1st costume from pentrails (you can write on the screen "HELLO WORLD" in red), make a new costume with background color of your choice dimension of the costume HELLO WORLD and combine 2 costumes into one !
Not perfect but it's a start !