In the past, I have asked this same question about how the McCormick card ended up with the white team letters, IE what caused this variation to occur. Whatever the reason, likely it is the same reason that more than 20+ separate 1969 WLs exist.
With the 69s there were multiple cards that ended up with the WL variation, while the McCormick is the only card in the 1968 set with this variation. The McCormick card is obviously far more rare than any of the 1969 WLs, and generally more valuable than any of the 69 WLs other than the 69 WL Mantle.
With the 1968 Cox and Brinkman cards, the color differences on the team name are ONLY due to the fact that both cards were part of the Milton Bradley Win a Card game/set.
https://www.net54baseball.com/showthread.php?t=165264