Since acu (Okay ucp or whatever it's really called) is showing up cheap has anyone dyed or painted it? I have seen some photos of dyed RAID packs from back when they were practically giving away the acu ones. The end up looking kind of muddy when dyed with RIT brown, which is fine.  Not sure what RIT would do to packs and pouches with padding. Is the the dye going to continually leach out of the padding? Spray painting seems like it would be easier but will it just crack and come off?
 

It is possible to dye a light colored object to a darker hue, nigh impossible to dye a dark colored item lighter.

You select the color you want to move towards, and set up a tall stock-pot with plenty of water.  Heat it up to boiling, and add dye, stirring.  Some folks add a couple of cups of salt, claiming it helps the dye to take better; likewise some add some rubbing alcohol.

With nylon, unlike cotton which always darkens when wet, what you see is what you get.  Dunk the ACU item and let sit, pulling it out and reversing it while inspecting for right shade.  Once you're there, yank it out and rinse off.  If done right, and rinsed very thoroughly, the item will be permanently dyed with very little residual bleeding of color, if there is any at all.

Rit dark green seems to work pretty well; the rit brown seems to come out rather reddish.  OK if you're invading Mars, I suppose, or if you are slithering through red Georgia clay a lot.

Some folks use camo Krylon paints to color-in their packs.  The most successful technique seems to be several light mistings of the paint, applied one at a time, and allowed to dry.

There's a number of threads discussing this; I'll look around and post a link here.  OK, found what I was looking for.  The references––and links to other dyeing threads–– show up on pg4, but there are a LOT of recipies for dyeing in this thread:http://www.ar15.com/forums/topic.html?b=6&f=10&t=270564&page=4

The other thing I read somewhere was that a lot of the stuff like packs won't take a dye because of the waterproof coating and I guess you risk compromising that (even though the coating is temporary) with the dye job. Not a concern for me but you also possible effect the IR resistance coating stuff. Paint may be the way to go.
 

IIRC, someone did a check on Rit-dyed stuff with IR/NV devices, and the Rit dye did not add to the IR/NV signature of the item.  Thing is that to dye most packs you're going to need a huge kettle/vat, and that is not a common item.  For large items, misting on some layers of spray paint is probably the way to go.

Most nylon material has a waterproof coating applied to the side of the material that is almost always on the interior of the product.  This being so, the coating has no effect on painting/dyeing.  If, in some unlikely event, the exterior of the item has been sprayed with Durable Water Repellant, or something similar, that is another story.  Might be a good idea to e-mail nikwax (the mfr of such DRW sprays) and find out how to remove such coatings.  I'm sure there is some sort of solvent that will strip off the DWR without harming the underlying material.