I'm with you, I like the look of old deep look brass. I have learned over the years to polish them up to perfection, then over time keep them rubbed down and clean. It takes about a year to get the deep golden color back and then only gets better after that, but the KEY is keep it clean from the acid in your hands, some of us have high acid skin.
A way that you can speed up the process ,,,,, KINDA ,,,,, is polish up the grip frame to clean and perfection, place about a inch of vinegar in the bottom of a plastic coffee can, suspend the grip frame in the airspace above the vinegar, cover and let it sit for a few days. This method us used to "patina" brass parts, I have used this method before to age black powder parts on a muzzle loader to get the aged look.
Results are a neat looking molted look, some greens pop out, black too, wash part off good then some gun oil and a gentle massage in some of the right places.
What I understand is, it's oxidizing the metals. I would suggest you find some sort of brass part and try this before doing it to your grip frame.
To me freshly polished brass just does not have the depth of color that slowly over time aged brass has. IMO