I am currently considering the same problem. Last month while visiting in Massachusetts I saw a GB 42 with painted teak and oddly, she looked damn good. (somewhere I have a some photos)
The teak was painted a very very light greenish blue that contrasted with the white deck house. The top hand rail was still bright (The one on the stanchions attached to the toe rails) Even the transom was painted and looked good with bright name and hale boards.
For teak, I do like Cetol and finish it off with 2 or 3 coats of gloss Cetol.
'73 GB 42 CL #362
Punta Gorda, Florida