I just bought a bushmaster upper to put on my mega lower.

(Megas are the same color as RRA)

It looked fine until I mounted it. compared to the mega, it looked almost blueish-greenish-black, and doesn't look very good.

I had considered trading the upper receiver for a RRA, but the tools are prohibitively expensive.

So I decided I want to put some type of gun coat on it to make it darker. I didn't see any threads on this, so if somebody could point me in the right direction, that would be great!

Here are an upper and lower I put together just for demo purposes.  The old A1 upper is refinished in Norrells Flat Black Socom.  The lower is a DPMS with factory finish.

You literally cannot tell a difference in finish from the upper to lower.

Quoted: Take a look at the various Norrells threads, but remember that if you are looking for a perfect match, you will need to do the upper and lower.

Yup.

Norrells.

Period.

Duracoat it. A little expensive, but in the end it's worth it. If done right, it will last for ever,can be any color(or "shade" of black) you want and with infinately repeatable color matching,ie. scopes,rings,bi-pods,etc. even if they are added in the future.