Nanuq's trick might actually work. It's just that I don't like artificial solutions, and fake patina color is just fake patina..
Try to bake it in the oven for 1 hr at 220 degrees.
On a UK website there have been guys experimenting with this, some have been leaving it in longer and getting different colors on both pearl and insert, a nice brown can be achieved with longer baking, I've done it with 30 minutes at 250 degrees for my 5514 COMEX build and got a nice vintage pearl effect without damaging the insert.