Replacing / selecting all pixels with the same colour

Hi I'm trying to use the nice animation feature of Realworld Paint to make this sort of filmstrip for 2d videogames:
http://s5.postimg.org/mzrcgugxj/jetpaco_spriteset.png
What would be the best way to make the mask frames? The ones with background in red.
Is there some option to select all pixels in red so that I can past them into a new frame?
Is there an option to crop or fill in black all except what is in red?
What else woud you recommend to make this type of work? Thanks