According to Swearengen, they started making 97 Riot guns in March of 1898. 1908 was the transition year from a high comb stock with a round bottom English style pistol grip to a fluted comb, flat bottom pistol grip.
There were military 97s earlier than WW1, but they were Riot guns and not trench guns. They were procured for duty in the Philippines, and Canfield says there's no guarantee all had military markings but at least some did. The guns were procured from about 1900 to about 1910, and serial numbers of known specimens had a C prefix and ran from about 150000 to about 200000.
Nothing I have seen indicates any were finished in other than the factory blue.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.