sorry not to jump on this immediately.
Being first isn't always being best. I find the best bowl of chili is the last from the pot after it has simmered all day.
This stamp was among the workhorse stamps of the 1910s and early 1920s, produced in huge numbers including the stresses of wartime printing. It will vary quite a bit in shade in addition to the difference among flat, rotary, and offset printings, and the changes due to a century+ of storage and handing. Bottom line, I would not get too caught up in the minor color differences. You have the perforation rate and the watermark determined, which is a huge way toward a final ID.
The perfin is WP/Co, note the top of the "C" under the W, and of course the lower case "o" does not show at all.
Attributed to Pyrene Manufacturing Co of New York City.