When I posted the FTW Blogger Spotlight on The Back 40k, here's the reply I got to that question from SandWyrm:
For the love of god, don't prime your models with black!!! I don't know how this stupid tradition got started, or why it lingers, but it's awful advice. Use white, gray, or brown instead. I use a brown wash over a neutral gray undercoat myself. The brown enhances details without sucking the life out of the colors you paint over it. Many people wonder how I get such a bright look to my army. That's how.
Image from The Back 40k
He's very passionate about this and for good reason, just look at his army and you'll see what using a color other than black can do. He's even posted a step by step on how he paints his tanks.
So, I figured I'd look into this idea a little more.
Like everyone else, I've always used black. Maybe it's just the color we all start with, who knows? The Painting Corps asked the questions once, What primer color is best? I'll be honest, using black makes it easy for me. Black always looks like a "shadow"and it makes it easier when you don't have to paint all the way down into all those recessed areas on the model... just leave them black and call it a shadow.
But that easieness comes at a cost. When you prime with black, it can take some of the life from your colors or increase the amount of work you have to do to get those bright colors you want back. For some of us, this is acceptable.
Just getting our armies painted beyond priming can be a struggle. Berks Warhammer 40k has got a great article on doing just that, getting your guys tabletop ready and a follow up artcile on finishing them off.
I will say that there are times when black is the color to use. Some paint schemes just lend themselves to using black.
Over the past year, I've done a tremendous amount of painting. I've made a conscious effort to use colors other than black when it comes to priming.
I first experimented with different colors when I painted my Lustwing army. For these guys, I went with a rich, redish brown color. In the end, it paid off in spades, giving the army a unique color and look on the tabletop. As for those wondering what that redish brown color looks like, it's the color of the trim around the edge of the base. I also kept the primer color as the base color too but that's another post.
My next real chance involved 20 Adeptus Custodes. These guys are all gold and using black as a primer would have been a disaster. Fortunately, my Client let me use a dark brown as the primer and basecoat for these guys and they turned out better than I thought they would.
When it came time to paint my current Deathwing army, I opted for a light grey primer and basecoat. Of course I tried a completely different painting style as well, but I don't think the army would have looked as good had I started with black primer.
I'm working on a Space Hulk commission right now and I'll admit that I was a bit worried at the idea of using something other than black to prime. I know it will make the models look better in the end, but could I pull it off effectively was the question. While I've done it before, it's still out of my comfort zone to some extent.
To this, I can only say give it a go. You'll never know until you try it out. The Genestealers don't look half bad so far.
UPDATE: Since this post is fairly old, I thought I might add a few more links to help expand on the material covered in this post:
Priming: Things to consider
Priming: The actual process I use when I prime my models