Step 7: Painting
For Batman, gather some black, gold, grey, and blue. For Robin, you'll need green, gold, red, and black. For Spiderman, white, black, red, and blue. For all of the gnomes, you'll need some colors for the beards, and some browns and greens for the shoulder bags and foliage.
There is no need to prime. Just start painting. Another thing to consider is the costume. It's better to roughly follow the gnomes existing clothes than to crowbar in an exact interpretation of the comic book costume.
Download the characters' emblems from a Google image search. Use these as a guide to sketch out the emblem onto the gnome's cap using a soft pencil. The cap is the most prominent piece of clothing on a gnome. So it is best to put the emblem on the cap rather than, say, the chest. Once it was sketched out, paint the emblem in with a fine artist's brush and acrylic paints.