I'm a regular poster at Roderuscustom's Home Gunsmithing Forum. I have extensive experience (for an amateur) in anodizing and dyeing aluminum, as well as Parkerizing and hot blueing steel. There is a wealth of info about DIY finishing at Roderuscustom, go look.
I don't understand your goal; by "bright" do you mean glossy? or do you mean polished bare aluminum? You also mention "green" do you mean an olive drab color finish?
The original and best way to finish aluminum on an AR15 is anodizing and dyeing. PAINT will always look like paint, and standard anodizing provides a surface several thousand times harder than any paint in existance. The corrosion resistance of anodized finishes is also outstanding. The usual and recommended surface prep before anodizing is bead blasting. This will provide a dull to semi-gloss finish depending on glass bead size. Caswellplating.com can sell you various shades of green anodizing dye, but green is a tricky color to dye, be prepared to experiment on scrap first. Forget RIT or any other fabric dye, get dye that is specific for dyeing anodized aluminum.
Another thought; semi-gloss black anodize looks interesting on an AR if the upper and lower both have it, dull olive drab is also cool, but a gloss or semi-gloss olive green may look cheezy.