I made this:
Gears of War lancer paintball gun | Gears of War 3 Blog
I also made a prop version (follow link in my sig), but I've made improvements to the paint job since then = Rub N Buff.)
I was going for a GOW 1 look rather than 2-3 because the first game's gun was more gritty and colorful. The GOW 2-3 Lancer is pretty monochromatic and boring IMO.
Anyway, if you like the paintjob, here's how it works:
Silver spray paint the whole thing.
Use a dark desaturated brown spray paint for the foregrip.
Spray spots at a time with flat black and rub it off with a cloth (preferably one that doesn't leave little fuzzies behind. This will let black cover the recessed areas and let you rub back to the silver on the raised areas. You can also spray more silver in spots as needed or dry-brush it on. Work this process to get a worn metal look.
Get Rub n' Buff silver and black. Use the silver to really punch up the metallic look. Use the black to create darker metal colors. I also used a hint of blue Rub n Buff in some areas.
Blood: Get a Tamiya paint color called "transparent red."
Also get a dark reddish brown color. Use both of those together to get variation between old dried blood and fresher blood.
-That's how I did it. Hope that helps.