Replacing / selecting all pixels with the same colour - RealWorld forums

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

Here is some info about a script that should be able to do what you want.

Thanks.
Is there an option to get transparency mask ?