Hi, Chuck
Anytime a UP related question comes to mind, "Utah Rails" site is one of the first places I visit.
http://utahrails.net/pass/pass-paint.php
(scroll down for a discussion on the use of two-tone gray)
Once there you can further search to see if there is any more details.
Good Luck, Ed
When did UP start using their two-tone gray paint scheme on passenger equipment? And how wide-spread was its use?
ChuckAllen, TX