It's advertising referencing the Cooper Cycle Company City Agents who ride White Flyers which are a type of Barnes Special bicycle and which evidently were painted white back in the day.The Cooper Cycle Company advert is centered below the map in the margin.
The Union Crackajack was evidently a Union bicycle for real "Crackajacks" or a riding group by that name and they chose to ride the "Barnes Special" which evidently was painted white as noted in the other advert and according to the description was well-made for the time period in that welds at frame joints were ground down so there were no obvious seams at the connections. It apparently was a quality product.
It looks like several of the bicycle ads reference specific colors for the brand they advertise so that may have been a distinctive maker mark from back in the day.
For example moving around the margin from UL corner - Fenton bicycle described as Blue Crown (maybe a trademark); along UR margin - March-Davis Cycle Company was the "Speedy Pink and Blue"; LR margin and LC both mentioning the Barnes Special; and I suspect that the left margin advert for National and Deere Implement Company bicycles were distinctively colored.
Just my guess.
EDIT: As a matter of fact I found a 1900 Barnes "White Flyer" Cushion Frame bicycle [0] listing in a UK museum site.
That description supports my guess that each manufacturer used color to distinguish their products from the competition. I got lucky.
Here's a little more history of a bicycle racer, Eddie "Cannon" Bald, who rode a Barnes Special and an example of the bicycle. [1]
There's also an eBay listing for a Barnes "White Flyer" frame [2] that is not cheap.
And finally, someone really knows their Barnes bikes and has a great example. [3]
[0] https://onlinebicyclemuseum.co.uk/1900-barnes-the-white-flye...
[1] https://thecabe.com/forum/threads/barnes-white-flyer-special...
[2] https://www.ebay.com/itm/163237177987
[3] https://thecabe.com/forum/threads/barnes-white-flyer-special...