i wont paint my guns black seems for more menacing than beige (spelling?). plus you can use a black gun with almost any uniform. desert woodland, urban etc. where as tan is primarily for desert. but for a paint job both guns look good.
Go into those environments you described with a black weapon and I will spot you nine times out of ten.
I used Tan and Brown in my gat, with a tiny bit of green, and the small ammount of black that's left. It blends great in the areas we play.
You are right, however, in stating that some rifles just look good black! CAG D Boys is one of them. Knights trades, AN/PAQ4, Crane stock, and a Knight's QD suppressor - it looks good!
I found some paint at ACE that matches Coyote Brown quite well. The color os called "Saddle Brown", and it's part of the "Home Accents" paint line. I'm going to pick up a can, and see how it works. I'm still looking for a color that matches Smoke Green/Ranger Green.