Note: these are unfinished, cast metal pieces (they may need to be sanded and/or have tiny holes filled in before painting or finishing).
You can leave them silver colored or repaint them. And painting can be easily done: just spray paint them!
And don't forget to wash them (just soap and water... but probably with an old toothbrush, too) to get rid of any oil or mold-release residue so the paint will stick well.
See the tutorial by Design Team Member Lora Mahaffey for more info on prepping and painting cast metal miniatures.