I have a standard Anderson AR15 lower that was SBR engraved and I just want to fill the letters to match the standard anodizing finish as close as possible.

What do you recommend?

Thanks for your help.

Aluminum Black or whatever it's calle.  Made by Birchwood Casey I think.

I've always just used a crayon, but I'm cheap haha

+1 on the Aluma Black from Birchwood Casey

You need to do it ASAP before the bare aluminum oxidizes in the engraving. Also be sure to clean the engraving before application - use a cleaner that does not leave any residual leftovers and a soft bristle brush. You may have to apply a couple of coats of the Aluma Black to get the desired finish you want. I let mine dry over night and then apply a light gun oil over the engraving to seal the deal. It may appear to be a deep dark grey color until you apply the oil - then it will turn black. I have done this on a couple of engraved SBR's with great success...

Ive always us3ed model paint to color fill in engraving on lowers. Get what ever color you want and some mineral spirits. CLean it with the mineral spirits and ince its dry go over the engraving with a small brush. Dont worry about getting in outside the engraving. Wait 5-10 min than get a small rag damp with mineral spirits and rub it over the engraved area but dont rub it to hard. You just want top take off the extra paint and leave the paint in the engraving. You may have to do it 2-3 times but itll turn out pretty good. Ill try to post some pics of what ive done before it turns out pretty good.

Might check out LaquerStik

 
You don't have to do it ASAP. Oxygen bonds to Al almost instantly, but it won't harm the bare aluminum, it actually helps it somewhat from corosion.