Ive read the Tuscan ones tended to be used more on the high profile, named passenger train routes and streamliners. And green had more freights.
I don't recall for 100% sure but I think the motor windings were different. (or the gears are in the motor case. It has a quill drive off the output shafts. A very unique drive.
From 1935 to 1955 GG1s were painted in either the Loewy or Modified Loery dark green, 5 stripe schemes.
In 1952 ten GG1s were painted Tuscan Red with 5 stripes and used for passenger service.
In 1955 saw the Pennsy switch to one broad stripe for both DGLE and Tuscan Red GG1s, plus three were painted silver to pull the Congressional. Not all GG1s got repainted right away and at least one 5-stripe survived to the Conrail era.
Many of the 13 red and silver engines survived into the PC and even the Conrail era. So there should be plenty of evidence these special passenger painted GG1s pulled freight but probably very little freight until the late 50s or early 60s as passenger travel diminished.
The remaining 126 GG1s maintained the green/1 or 5-stripe and pulled both passenger and freight, and I think primarily passenger until the 50s. I think this because looking through my books I see very few photos dated 1952/3 or earlier of GG1s pulling freight.
So, if you are like Arnold looking for late 50s/60s data I would think it perfectly acceptable to pull whatever you want behind any color GG1. However, DGLE with one broad stripe would be the dominant scheme for that era.
ADDED: I see Jonathan beat me to it.