Here's a step by step guide on how I'm painting up my new Imperial Guardsmen.
I was trying to match up the USMC's woodland digital camouflage pattern, but that turned out to be a tall order. I used a piece of posterboard to make a swatch, and compared it to the actual camouflage.
The black is easy--just use Chaos Black. The green was difficult, as Dark Angels Green wasn't quite it, so after lots of experimentation, I found that the best match was Vallejo Model Color Gunship Green. It had the right shade and level of saturation, but was just a little bit too light, so I darkened it with some Chaos Black. Then it looked a tad too desaturated, so I put in a few drops of Snot Green, but for some reason the Snot Green didn't mix too well with it. Regardless, the green is pretty close. The light/sand brown matches pretty well with Khemri Brown, so that worked out. Finally, the main color of them all, the brown... Now that was a pain. I made a separate swatch with nothing but shades of brown--Bestial Brown, Vermin Brown, Calthan Brown, Graveyard Earth, Snakebite Leather, Desert Yellow, Scorched Brown, etc., and none of them seemed to match right. The closest color (which is on the swatch) is Snakebite Leather, but as you can see it's not quite the right shade, and is way too bright. I messed around with mixing some colors, including Snakebite Leather and Graveyard Earth, and got pretty close, but it was a pain to do so, and since this was going to be a major color on the models, I didn't want to have to mix it up every time. So I settled on just using Snakebite Leather, and figured I'd tone down the shade with a wash of Devlan Mud.
So with the pattern down, it's on to a miniature:
After assembling and priming the model, I basecoated it with Chaos Black, Tallarn Flesh, and Calthan Brown.
I then painted the armor and boots with Desert Yellow, while I gave the cloth a coat of Snakebite Leather.
Then I gave the entire model a wash of Devlan Mud, to shade and tone down the colors.
Next started to paint the camouflage pattern. The MARPAT woodland pattern is dominated by the brown shade, followed closely by the green, with slightly less black, and a small contribution with the lighter shade of brown. As the real pattern is mainly dominated by the brown and green shades, I gave the green a significant amount of coverage.
For the next stage added the Chaos Black patches, and small dabs of Khemri Brown on the cloth to finish up the pattern.
Finally I highlighted the armor with Desert Yellow, highlighted the skin with Tallarn Flesh, painted in the eyes, and finished up the base. Voila! Now I just have to do all that a couple hundred more times! I plan on doing a similar guide for the ACUPAT in the near future.