CC,
What you are trying to model is a condition known as Vitelligo.
The "Spatter" technique could work if your "Cavas" was larger, but it isn't
It is a few sq. mm.
So, you need to get just a few areas that are either WAY lighter, or WAY darker.
Vitelligo also tends to crawl across areas as if a bug or octopus has crawled across your skin and left a track where it went.
Generally, on a small area like the Uruk-Hai, who do not have much flesh showing anyway, it will show up more on the legs and arms.
I have done mine so that only one or two limbs at most have the really visible mottling, and the others will just have some mild dis-colorations.
Instead of the "Spatter" technique, I will paint the figure with the usual base-coat for the flesh (in the case of the Uruk-Hai, it is blue for the Mordor Uruks and Violet for the Isengard Uruk-Hai. Then, I will dot some areas of either white or black in a random splotch/slash across the limb, with one area that is slightly larger than the others, surrounded by some smaller dots and blotches.
Then, I do the wash as usual (which in the case of the Mordor Uruks is an almost black-brown, and in the case of the Isengard Uruk-Hai, it will be a very dark red (almost black, usually with a slight amount of blue in it, which gives it a strange violet-brown cast).
Dry-Brush time, Mordor Uruks get a cool brown, and the Isengrad Uruk-Hai
get a more red-brown dry-brush.
Then, I will pick out the white areas again, but this time with a more cream color.
If I am feeling very obsessed about it, I will make sure that any vitelligo that crosses any folds in the skin, creases or deep areas get a shading with a peach color.
Then, just because Vitelligo is rarely JUST light areas that are blotchy on dark skin, I will do some very small dots and blotches of blue-black that surround the areas of white slightly at an orthogonal.
On the Berserkers you have a much larger area to go with. The Faces of Lurtz and the other Uruk-Hai personalities also give you some room to play. Be careful that you keep these areas small. It is easy to go overboard.
Also
ALL of the Orcs in LotR had Vitelligo. The Moria "Goblins" had very pale skin (white in some cases) that had dark patches. Some of them had a greenish tint to their flesh tones (more from the lighting than actually having green skin. I have seen the offending Orcs in natural light and in the set-lighting, and in natural light, they are very albino with grey and blue vitelligo (it can be blue as well, but think blue as in blood veins under your skin
break those, and you have a bruised skin that gradually assumes the blue color if it doesn't heal properly, or if vein formation is disrupted).
The Mordor Orcs come in a variety of colors, supposedly (according to the Weta design team) to reflect the different areas of Mordor and Clans they belong to. Obviously, because of how orcs are harvested/farmed (and I do mean farmed) you may get some mixing in the crops, or if an orc actually find someone (thing) to "Breed" with, it may establish some sort of genetic markers that can be traced through skin tone. All of a "crop (if they are "Bred" in the normal manner) will have the same general cast to their skin, and considering that certain offspring are seen as better "seed" crop than others, you will tend towards several dominant skin colorations.
In the Movie, this was reflected in a yellow, orangish, grey-blue/green (again, not really "green", but a skillful mix of blue and grey will get you a "Greenish" look that really isn't green), black, and white skin colorations.
The "Vitelligo" on each skin coloration is best a highly contrasting coloration. Black on white, white on black, orange on blue, blue on orange, and so on
As soon as I get moved, I will finally be able to get the pictures of the first of my LotR figures on photobucket. The Mordor orcs and Uruk-Hai all have some pretty good examples of vitelligo on them.