this is the acog i use in green http://www.trijicon.com/na_en/products/product3.php?pid=TA31F
with this mount http://www.americandefensemanufacturing.com/view/product/187/
stay as far away from those crappy larue mounts as you can. almost ruined my acog trying to remove stripped larue screws.
i prefer the 4x acogs with the green fiber optics they act as a rds and a magnified optic. the reticle i have is a 556 reticle but it works very well with 308 out of a 16" barrel on my scar17. i put it on my ar15 now and then as well.
the green is more visible than red in low light and against green backgrounds. yellow is the worst color for a reticle because it fades faster than red which fades faster than green. this works different with folks that are partially color blind...
your going to be hard pressed to find a better ar15 optic. the glass is amazingly clear you dont have to muck with settings once its zeroed and its damn near bullet proof, literally.