[BigJJ]

Back from the library.
I think we need to update the uniform references we use.
I found a number of pictures of Giants in both 1900 and 1902 wearing dark plain caps and socks, as in the Mathewson photo. I did not check other years.
There is a big picture of Mathewson, in uniform, wearing such a dark plain cap and socks in the New York Daily News on May 9, 1902. The article that runs with the picture describes an away game in Chicago. I do not know if the photo is a file photo, or one from the game. The photo was reprinted for another article on June 29, 1902, so it may be a file photo, but that was for an away game as well, this time in Philadelphia, so I believe they used the photo as a file photo for Mathewson to print for away games. The News from April 17, 1902 showed the team ready to play its first home game, and the team was in white caps. I believe the away cap was a dark in 1902. or a plain dark cap was an alternate hat for home/road, but it should be formally listed in our references. more research is likely needed.
With regard to 1900 (and 1901) unfortunately the NY library does not have the Daily News if you can believe it. so, for 1900, I was left with small pictures from The New York Herald. I couldn't post because the pictures from The Herald are so small, but I could see on the small microfilm, Jack Doyle in play on April 12, 1900 in a plain dark cap. I do not know if this was a file photo or one from the current 1900 season.
Unfortunately, there were scant pictures of the ads at the Polo Grounds, so it is not any clearer to me when in the 1900-1902 timeframe the Mathewson photo could be from. I'm including 1901, as we do not know what month the 1901 Polo Grounds photo we have is from, though the liklihood remains 1900 or 1902.