I agree that there will likely be a touch of difference. However, having a loco slightly different shades than the coaches -or even differences between coaches- is not necessarily unrealistic. Recall that SP had quite a few "Daylight" trains of different ages, running different equipment in different locales.
Someone may have better information than I, but it seems reasonable that differently aged pieces might end up on the same train. Perfectly matched passenger consists were not necessarily the norm outside of promotional pictures.
I eventually settled on all Athearn for my Daylight set (cheaper, shorter cars for my small layout), but at one time I had Tyco, Athearn, Con-Cor and Rivarossi on the same consist and it looked just fine.