Ron Burns wrote up the American hard paper in a recent article for the USPCS Chronicle. It will be online later this year. He postulates that with the merger in 1879 old stocks of the paper National had used in the late 1860s became available, and American used them a bit at a time.
Were there later National printings that used the ink intended for the Continental? Or did Continental print some stamps with National plates?
Why suppose that these stamps looked this way the day they were made?
Continental took significant measures to distinguish its products from those of National. Since 1861 Willian Ormsby had pushed for the creation of Continental because he did not respect the leadership at National who were willing to do business with the Confederates at the time. The use of their plates was initially a hot issue, but apparently by 1873 that had cooled to the use of color alone to distinguish product, since they had made no new plates from the upper denomination dies they had altered.
For the six cent they HAD produced new plates, and those make the distinction. The problem with color variation is that over time, and under various storage conditions, the color differences are less pronounced. Since neither of your stamps bears a secret mark, they both started out from the presses at National. The one on the right is not as fresh in appearance, and appears to have toned in the direction of a not-so-fresh Continental. I really don't believe there is anything more to it than that.