A little bit more on the development of the white'n'red livery...
It wasn't a foregone conclusion that these were to be the main colors of the new livery. What was decided was that there would be one main body color, and a second color at one end (usually the front or short hood end; there were exceptions), with all the below-frames stuff - trucks, fuel tanks etc - black. Remember one driving factor was economy; the livery had to be cheap to apply. Even the DSSA livery as posted by Steve1, was expensive to apply in terms of time, and masking.
The livery was heavily influenced by the then-recently introduced Southern Pacific livery of Grey and Scarlet, also known as the "bloody nose" livery for obvious reasons. However, Wallace Abbey took into account one major drawback of the Espee's scheme - in monochrome (black & white) photos, the shades of grey and red looked almost - or even absolutely - identical. See this link as an example:-
http://espee.railfan.net/nonindex/sd07_photos/5322_sp-sd7-fred_a_stindt.jpg
Spot the Red & Grey?? This'll help -
http://espee.railfan.net/nonindex/sd07_photos/2740_sp-sd07-bob_dengler.jpg
So Wallace Abbey knew he wanted the colors to contrast better. He experimented on some models in his basement. He did do a version in Grey & Red similar to the Espee's, and one in Black with Yellow, and one in White (actually - and always officially - a very light gray) and Red, and because he happened to have the color, Black with Turquoise!!!
The Light Grey(White) & Red got the vote of the Board, although there were opponents. White is, of course, a rather daft color for any form of land transport liable to getting dirty. The argument went that every greasy handprint would be visible, never mind the soot and dirt. Wallace's answer was that if you could see handprints, you were standing too close to the loco (especially if it was moving!!). His argument was that his livery was designed to be seen from miles away - and with that big "SOO" logo on the side, who could argue?
He also asked the most vociferous opponent of the white'n'red scheme, what color the man's automobile was? "White" was the rather embarrassed answer... (which Wallace already knew)
The slight sweep of the red/white join on the cabside was a deliberate attempt to give a slight sense of motion, or at least direction, to the livery. The cab side was chosen as the dividing point, as the livery had to work on many different designs of locomotive - some as yet unknown, but it was reasoned that whereas bodies might be full-width or narrow hood, grilles might be who-knows-where, and so on, they could be pretty sure that there would be an uncluttered cab side, with a window, on the loco and the divide could go there, with, in most cases, the loco number. The main exception was Switchers (as per the MP15 in the OP) where the whole cab was red. The opposite end of the loco also was painted red to balance the effect, something not easily appreciated until later when a variation was tried with a new slanted logo, the opposite end left white, and the number moved from cabside to far end of the loco just above the walkway:-
http://www.rr-fallenflags.org/soo/soo0801dsa.jpg
The disadvantages of this variation were soon apparent; the slanted logos were not of the best quality & soon wore off; the white end rather than red just made the loco look 'unfinished' next to the normal livery, and the number position was inconvenient, and often obscured altogether in the winter by build-up of snow on the walkway!! That variation was soon abandoned. One change was kept; increasing the angle of the red sweep on the cab, with a very small curve at the bottom, which became know as "hockey stick", as that's what the new line resembled.