Years ago there was a guide on the GW site. It's gone now, but I have the text of the guide, but no pictures. It's a really simple way to paint them and they come out great.
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Start things off with a hearty drybrush of Scorched Brown over the entire model. Use a
large drybrush and just go at it; don't worry about details at this point. Just get a good
covering of paint. Next, finish off the bases. Or don't! It's your choice. Personally, I prefer
to do the base early on as it simply gets it out of the way.
Okay, enough drybrushing; time to stipple instead. Paint all the metallic areas of the
model with a heavy stippling of Bestial Brown. We're talking shields, swords, helmets,
gauntlets, leggings, chestplates – everything that's metal! This coat of brown paint will
act as a layer of rust/beaten metal. Next, apply a solid coat of Scorched Brown to any
wooden handles or bows.
Time to finish up the metallic equipment. Go back over all the Bestial Brown areas you
just painted but this time with Boltgun metal. Give the armour, swords, and the like a
good stippling of Boltgun Metal. If you want a darker metal, mix in some Chaos Black
with your Boltgun Metal. You'll see how the rusting effect comes to life during this step.
Be sure to clean you water after using metallic paints, as tiny flecks will remain in the
water and spoil future colours. Once the metal is out of the way, finish the wooden parts
of your models by highlighting with Dark Flesh.
With a lot of the sloppy brushwork out of the way, it's time to concentrate on tidy
brushstrokes. Heavily highlight all the flesh areas with Dark Flesh. As you can see, the
earlier Scorched Brown drybrush has done a lot of the work for you. This fact becomes
very apparent when you next go over all the leather on the model once more with
Scorched Brown. Finish up by painting gloves, belts, straps, and any hair with Chaos
Black.
Highlight the skin with a mix of Dark Flesh and Bleached Bone. Start with only a bit of
Bleached Bone in your mix, then work your highlights up slowly. With the flesh taken
care of, move on to the leather. Grab Scorched Brown and once again mix in some
Bleached Bone. With this mix, highlight the edges and rises of the Uruk-hai garb. To
complete your basic Uruk-hai, mix together a bit of Codex Grey with Chaos Black and
highlight the edges of all the black leather and any strands of hair on un-helmeted Uruks.
At this point, you can add a few finishing touches to your models if you wish. Gore
spatter can be accomplished by layering Blood Red and Chaos Black followed by Blood
Red onto weapons, faces, and armour. Another easy detail that adds character is The
White Hand of Isengard. You can accomplish this symbol easily by slathering vaguely
hand-shaped Skull White icons on faces, shields, chests, or wherever. Finally, you can
pick out teeth with Bleached Bone or a dingier colour – after all, Uruk-hai aren't known
for their oral hygiene. So there you have it: six easy steps that get your Uruk-hai force
looking good in but a few evenings of painting.