The hoppers are for MOW service. Guilford painted more gray boxcars than they painted white,only one entire series of boxcars was painted white around 1985 (the B&M 79000's) which was about 100 cars,and a few 3200 series B&M PC&F cars,(later one SIECO car), no more than about 10.
One of these was the LIVE FREE OR DIE boxcar, BM 3249, which now sits out of service at Lawrence Yard. One D&H boxcar was painted white also, D&H 27073. Many more Maine Central boxcars were painted white, most of these were the 27-29000 series "waffle side" cars, about a dozen FMC cars, one PS-1, one 35000 series Berwick car,and at least one double-plug door car, most of which are all now retired from regular use. Several hundred MEC boxcars however, were painted gray. These were the "bleeding G" cars, which railfans chided Guilford for the runny paint on the white "G", what they don't know is that these cars were not painted by Guilford, they were painted by GE Railcar in Sayre, PA. in 2004. A few other MEC cars were painted gray before that at the Waterville shops, even at least one waffle side car, but the paint did not run on these cars, though it did fade more quickly then the GE painted cars.
Hope that's not too confusing!