I think the blue scheme came in around the mid 60's, before the 1970/1 merge of GN, NP, SP&S and CB&Q.
Even though HO is 1/2 O gauge, when considering 3 dimensions, weight and volume is dramatic. ie, a 2X2X2" cube is 8 square inches and a 4X4X4 is 64 square inches so bulk and weight increases significantly when doubling dimensions. That's why the grand kids can play with the lionel stuff but not the HO!
Each scale has it's advantages. GN, being in the sparsely populated area of the north west, didn't get all the publicity that the eastern and west coast California railroads got, so modeling GN in HO gave me the largest selection possible.
Until the last 10 years, or so, there wasn't much made in O gauge. Not until MTH and 3rd rail started making GN steam engines. Lionel, for the most part, has only slapped on a GN decal on a steam engine and called it GN when GN never even had such engines, ie: hudsons, yellowstones and other steam engines that didn't have a belpaire firebox on it.
My O gauge GN steam:
Except for the 3rd rail M-2, all MTH:
As I like to say 'GN and NP were joined at the hip' and GN leased engines from DM&IR so I also have a 3rd rail NP Z-5 yellowstone, Z-8 challenger and a MTH DM&IR M-4 yellowstone.
