What state color Yellow means in shoulderboards (Kingdom of Finland)?

[Verikauha]

The title question.
These clearly are german made, but to be more specific, Finns ordered these from germany in 1918 after 2.4.1918 for their first uniform model.

In Finland we call these m/18 or M/19 shoulderboards, both used german origin manufactured insignia. Finnish made shoulderboards "copies" have clumsy-made look. Finns put rank Sterns to their collar, not in shoulders. Later these evolved into m/22 shoulderboards.

But when Finnish 27th Jäger Battalion (Königlich Preussisches Jägerbataillon Nr. 27) returned to Finland after 3 years of service, about 1500 men (there were total 1800) were promoted to NCO's and Officers, were vital in Finnish civil war agains reds and later formed future Officer-core of the Finnish army.

Those JB.27 men used and brought Prussian state colors black or black&white in their shoulderboards.

Thus this yellow state color confuses me, do you have explanation?

But I do have speculation.
In practice, Finland became vassal state of germany between May-November 1918 and Finland was know as "Kingdom of Finland" (May 1918 - March 1919) and Finns voted german Prince Frederick as elected King of Finland on 9 October 1918, but he renounced the throne on 14 December 1918. Because German defeat in ww1.

Left red is HQ or artillery, right black is engineer/pioneer shoulderboars. Lions are german made.