Which is better - black or white?
Rather than have two tins of undercoat fight it out, I thought that I'd try an experiment out on 
Editor in Chief Bill 
's Scrunts that he sent me to paint. I know that many figure painters get set in their ways and swear by either black or white (or grey, or cream, etc.) as the best undercoats. I always use black, but used to use nothing but white - I find black easier and therefore wholly preferable, but it's nice to confirm this by painting the same models in both.
I sprayed eight of the Scrunts with Citadel Black and eight with White. Initially after the spray, the white is done - no more messing about. The black models, on the other hand, need patching up and re-undercoating with watered-down Chaos Black paint.
Hmmmm, and I thought black was easier. White wins this stage.
Painting the robes, I used exactly the same colors on all the models. They are Bleached Bone with a Gryphonne Sepia wash. Again, the white took one coat of color and was done, the black models needed about four coats of thinned bone color to conceal the undercoat and give a strong color.
White wins again - so far, they are proving much easier to paint.
Also at this stage, any spillage of Bleached Bone on the black models needs to be patched up with more black paint. On the white models, mistakes are not so important as they will be covered easily by the other strong colors.
Another victory for white.
The bone cloth is highlighted with Bleached Bone, and then Bleached Bone mixed with Skull White.
Next, I applied Scab Red to the shoulder pads and helmets. Again, the black undercoated figures needed more coats of paint than the white.
I begin to see a pattern emerging here...
The armour has Warlock Purple as a highlight, with Tentacle Pink added for the finer line highlights.
The metallic areas were painted straight over the undercoat, either Dwarf Bronze or Chainmail. The bronze gets a Devlan Mud wash, while the Chainmail gets a Badab Black inking. There was little difference in the ease of application between the two undercoats. I was somewhat surprised that they looked the same on both, as well.
It is worth noting that the washes are especially important on the white figures, they help a great deal in outlining the separate areas of the figure. With black models, this has already been done for us as there will invariably be black 'shadows' between areas of the models. With that in mind, I suppose black just wins out here.
The skin is Bestial Brown with a Devlan Mud wash, then highlighted with Dwarf Flesh. As with the metal areas, on the white models this would look awful if it wasn't for the brown wash. The black figures were much easier here, I found that I could just paint the Scrunts' noses and leave the rest of the faces in shadow.
A narrow victory for black here.
The beards were painted in three different colors, using the same process on all the models. The blond beards are Bronzed Flesh with an Ogre Flesh wash. The grey beards are Codex Grey with a Devlan Mud wash, and the dark brown beards are a mix of Chaos Black and Scorched Brown. All have a bit of Bleached Bone mixed with the basecoat for the highlights.
There is no discernible difference on the white or black undercoated models.
All the pouches are going to be a dark tanned leather, which is Chaos Black mixed with Dark Flesh, then highlights of Dark Flesh, Vermin Brown and Vomit Brown. On the white figures, I painted all these areas black beforehand, which on the black models obviously wasn't necessary.
This makes this bit way easier on the black undercoated models.
The shades and gloves are highlighted in Fenris Grey, and then Astronomican Grey. The only difference is that the white figures required them to be painted black first.
The bases are painted sand, which is drybrushed with three or four layers of successively lighter brown over a black basecoat. It is a real pain painting the sand black on the white figs; on the black ones, I made sure I sanded the bases before undercoating them.
It is officially easier to base black undercoated figures.
To sum up, looking at all the figures, it is very difficult to tell which ones started black and which were white, the end result is much the same. The process of painting them had benefits and drawbacks for both. With a black undercoat, the basecoated color requires more application, but on white models lots of 'blacking in' is necessary.
I can honestly say that there is little to separate them. I recommend you use what you're used to!
To see more of my painting, look up pro-painted-miniatures.com.