I've painted quite a number of 28mm Zombies over my time as a wargamer, enough that I've gained a reputation for bringing zombies into every game I can. For the most part, they exist to provide cannon fodder for my Warpath: Firefight Plague army, but having zombies kicking around tends to be handy for a variety of games. Naturally, I have gotten painting the undead down to a quick process now, so I thought I'd share it here. I'm not the best painter out there, I refer to my standard as "Tabletop Plus", meaning it looks good on the table and pretty decent in your hand, but I'm not going to win any awards. Painting can be a daunting process for newcomers to the hobby, so I hope this demonstrates that you can get a decent result without too much hassle.
I started with a couple of Heroclix I picked up cheap, Caliban and Machine Teen. Machine Teen got a Mantic Zombie head and Warhammer Ghoul arm to replace what he came with and both were rebased onto 25mm Flat Rounds. I flock before I undercoat, but just do what works for you. I also use GW Chaos Black spray on almost all of my miniatures, but whatever works for you is fine. I use a brush and black paint (Formula P3 Thamar Black) to touch up anywhere the spray has missed.
First step is painting the flesh areas of the zombies, I've used GW Karak Stone, but any pale Khaki or Buff will do. Don't worry too much about being exact, so long as the flesh is covered, any mistakes can be fixed later.
Next up is a flesh wash to tone the skin areas. I've used GW Riekland Fleshshade here. Make sure to get good coverage at this stage, but don't panic if you don't, there's plenty of opportunity to fix mistakes later.
Next, put a green wash over the flesh, I've used GW Beil-Tan Green. Make sure to let the previous wash dry thoroughly, otherwise the two have a tendency to dry with a chalky finish that ruins the effect. Unlike with the flesh wash, this one can be a bit patchy, as it helps create the illusion of sickly, rotting flesh.
Next up, paint the clothes. I tend to use bright colours on my zombies because it helps contrast the flesh, and they still pop after the next wash. This is the one stage where a steady hand can help, as touching up the flesh can take a while. I also tend to avoid too much green or red, as these can be lost on the final product. As one of the zombies is holding a bone, I took the opportunity to recoat it with Karak Stone so it doesn't look like flesh, and did the same with the hair, using Formula P3 Crxy Bane Base.
The final wash goes on all of the clothes, hair and bones, I use GW Agrax Earthshade, but Army Painter Strong Tone or a thinned Reaper Paints Brown Liner would work fine as well. Don't panic too much if some gets on the flesh, it's unlikely that anyone will notice on the finished model.
The only real fine detail work comes now, taking a fine brush, highlight the eyes, teeth and bones on the miniature. I used GW Ushtabi Bone.
At this stage, I painted the bases, but when and how you do that is up to you. I do a wetbrush of GW Steel Legion Drab followed by a drybrush of Karak Stone.
The "finishing touches" are done with Games Workshop effects paints, Blood for the Blood God as Blood, Nurgle's Rot as vomit and a coat of Lamenter's Yellow on the eyes to give them a sickly look. While less tends to be more with the vomit and blood, they can be used to cover up any gaps or mistakes. The final result isn't prize-winning, but it looks good enough for most games.
As you can see, en-masse, the quick zombies look pretty menacing, even if the individual result isn't something you'd see in a magazine. Using the above method, you can crank out a huge amount of zombies pretty quickly, and it works on 15mm and 20mm Zombies as well.