I have a copy of the book which I purchased decades back ...... the dust cover was not in color at that time, so the one you posted is fairly recent and probably just someone's idea of what it may be. That being said, I think Lil Belle's paint scheme is probably fairly accurate.
One of the Groups I belong to has had files of early paint schemes ... try "Early Rail" though I am not sure that's the one. Narrow-Gage and Shortline magazine has been running some loco plans along with a color rendition by an artist, so check them out.
Also you may check photos of the two engines at the Golden SPike park in Utah ..... they were researched and the current paint is supposed to be pretty close to original.
Skip Luke
Switchman (ret): Grand Canyon Railway;
Engineer (ret): Georgetown Loop RR Inc., Sumpter Valley Ry., Monticello & Sangamon Valley;
Dispatcher(ret): Illinois Central, White Pass & Yukon, Burlington Northern Ry.
Mariner, Musician, Miner