I'm guessing you're painting the calipers on the car, and not removing the brake lines so you won't have to do any bleeding.
your biggest hangup will be pushing the caliper piston inwards so you have enough space with the new pads to fit on the rotor. Depending on the condition and age of your calipers it might be easy to do by hand or hard with a c-clamp.
I would try, before you do any painting, to push the pistons in fully on all calipers. Be sure to open the master cylinder resevoir cap and you might need to crack the bleeder valve at the caliper. That'll give you an idea if you're calipers are in ok condition, if you can't push the pistons in fully you most likely won't fit the caliper with new pads on the rotor. If that's the case, get some new/reman calipers first and paint those instead- so sense putting work into a bad part. The actual brake pad change is easy, it's fitting a used caliper with new pads over the rotor.