I'm working on an A2 build using the PSA M16A2 clone lower.  It's a pretty dark black.  I purchased a JSE A2 upper.  It's nice and looks good, but it not as dark as the the PSA lower.  It's more of a gray then a black.  I tried to use CLP, and let it sit for a few days wiping a little at a time with a black microfiber cloth, but I don't think the upper is taking it.  I've read that type III won't absorb anything.  Not sure what JSE used.  Are that any options for darkening the upper to get a better match.  Don't get me wrong, it's not terrible now but definitely noticeable.

Years ago I set up a home anodizing set, and read up on the process.

Essentially, a microscopic “honeycomb” layer is created on the surface.  

We followed anodizing with a cold-water wash to get rid of the acid.  Then, we immersed the part in a dye (was it called negrosyn?) - and brought it up to a boil.

Boiling supposedly “sealed” the honeycomb matrix.  

Try Caswell plating for dye - but I doubt anything will permanently re-color sealed anodizing.

No harm in trying though.

The only thing I've found that will slightly darken any anodized parts is dirty diesel motor oil. Wear gloves, rub it in and then rub off as much as you can. You will eventually clean off that black soot from the used oil so it's not exactly permanent. Contrary to what some will say, CLP just does not darken parts worth a hoot.