There are a number of somewhat recent threads about this, so I'd suggest doing a search as there's a great deal of information about these. There are a number of different opinions from missing ink to sunbleached to chemical reactions (from scrapbook glue or purposeful bleaching). Each example of this would yield clues that one could use to determine the potential cause for the color change.
Here are two examples I recently aquired. Some might say these are missing a color run such as red while others might say it's a chemical reaction from the heavy glue used on the back to place the cards in a scrapbook. Whatever the case (draw your own conclusion), they're neat cards to find and there's a good deal of them out there.
