If you zoom your image up real close you will see the top example has a slight color misregistration. That alone could result in the blue/purple color being contaminated and thus appearing as if it is more purple than blue.
Stampvirgin also offers a plausible explanation. The top example is a modern use (looks like 2008) on a stamp that was issued in 1973, so some 35 years later, any number of contaminants or exposure to light or moisture could have affected the stamp and ultimately contributed to the color change.
[edit]
Just for argument sake, I went to my stockbook and pulled two examples of Scott 1475 that I have:
Upon closer inspection, you will notice the top stamp has a missing perforation at the lower corner; the bottom stamp has a color mis-registration especially noticeable at the bottom of the stamp also.
Here's a close up of both:
The point is that stamps from this generation are noted to have a number of flaws that occurred in the production process.
Posted 12/29/2011 4:13 pm
