Can’t give you an exact date, but at some point BN did switch to a smaller logo. Maybe c.1990?
BN didn’t paint everything Cascade Green. Open hopper cars and gondolas were black with white lettering, and covered hoppers were often gray with black lettering. Not sure about brown cars - could be cars they chose to reletter without repainting completely (like say from the Frisco merger?)
Two old ore cars were painted black. The rest of the low side ore cars stayed GN or NP until converted to taconite cars or scrapped. Taconite cars were oxide red and yellow.
BN cars went from large logos to reporting numbers only to small logos. Mineral brown for covered hoppers came in the early 90’s.
There were also the bare metal hoppers with two or three panels painted green with a white logo.
I remember seeing about 25 of them come out of the venerable old ex-Pullman Standard Trinity plant in Butler PA in the mid-90s, right before they shut down once and for all. I think it was down to just the paint shop and fabricating end panels for covered hoppers by then.
There were three common BN lettering schemes. The first was the “big white BN logo” one you mention.
The second was the “logoless” scheme–very plain. It showed up in '86 or '87, though Frisco repaints MAY have shown up in this scheme earlier. I believe there’s also been talk that it was a leaser scheme. It would be interesting to see if there were any non-lease new BN cars delievered in the first scheme between '87 and '90.
The third was the “3 stripe” scheme arriving in January of 1990.
As far as I know, all BN hoppers were painted black, except for company service (MOW) cars. Which were usually brown.
The two sides were different too. The reverse side would have a green panel on the extreme right panel (to demarcate the rotary coupler) and the extreme left panel left of the herald was unpainted.