Another thing that helps in keeping the paint color even is keeping the
paint agitated in the cup. The best way to do that with a gravity gun
is to get a "devilbiss bag kit".
They make a kit with universal adapters. Then you can install zip-lock
bags in your cup for the paint to be "sealed" in.
That way you can shake your gun regularly and keep it mixed.
I even lay my gun on it's side in between coats.
It makes clean up a lot easier and you can even spray upside down
with this system. It's about $25.00 and sold at most gun stores