If you’ve looked at enough T206 scans, you’ve probably come across some Hindus that look a little weird. Many, like this Beaumont above, were printed with very faint red ink. Others appear to be missing red altogether. This phenomenon is most pronounced on cards with red backgrounds, though it does affect other cards as well. If you take a look at my example scans, you can see that most of these cards appear to have an orange background. Some have more red than others, but all of them are noticeably more orange than a typical example with a different back. 
If you take a close look at other cards with Hindu backs, you can see that red is missing, even when red didn’t figure prominently in the card’s design. 
These two Bradley portraits have a subtle difference; the color of the lips.
I have never heard any theory about why so many Hindus are missing red ink. It would stand to reason that quality control was not a huge concern at American Lithographic Co. where the cards where produced. After all, the cards were to be given away for free as advertising. That said, the overall quality of production across the entire set is quite high in my opinion. Most cards are well-centered with nicely aligned fronts and backs. Colors and shading can vary a bit from one example of a card to another, but rarely to the level we are looking at here.
Here’s my theory: Hindu backs were produced early in the T206 production run. My guess is that at the beginning, quality control was a little bit lax. Most of the Hindus were probably printed, even though the red was not as dark as they wanted. Once the cards were released they became a cultural sensation. People loved them, and as a result the American Tobacco Company may have wanted to ensure that subsequent printings would be of higher quality. It’s also possible that the printers didn’t realize that the design called for more red, because they hadn’t seen enough copies of the cards to be able to tell what they were meant to look like.
