I do know that the canon provinces of the empire have official colors distinguishing each province among the empire military.
The lack of blue is easy. Blue is Middenland's colors.
Why Ostland has black and white as the chosen colors is something I don't know. The provinces, except Middenland, and the Moot, use a color combination among their armies.
The only other provinces to use black are Sylvania and Averland.
It looks like the creative teams got a bit weird about the colors. Ostermark has purple with white, or yellow. But then you get black, which is a difficult color to get. Yet, Ostland, the notorious poorest province has black on it's regiments. And Sylvania, which has a twisted and isolated eco-system, had black on it's regiments.
Averland is weird for having black on their regiment uniform. At least their rich, so it isn't too weird.
Mareinburg has cheap colors in blue, red and yellow. Red is just abundant to get a dye for. We know from the blue woad paint that blue isn't too rare in the empire, so it's a common color. Yellow is a bit special, but it is needed to get green, so that needs to be common in the empire.
White, red, blue, and yellow are likely the cheaper colors in the empire!
@torroar, where is Ostland collecting their black dye for soldier uniforms from? And where is Ostermark getting their purple dye from?