Something I've noticed is that certain pieces appear to be slightly different colors for other pieces of the same color. In particular, hinged red pieces seem to be the most noticeably different, especially some versions of the red hinge plate.
As you can see, half of the hinge plate is quite a bit darker than the other half which matches the regular plates. The 1x2 fork clip piece is also a little darker, though not as much as the hinge plate and it's hard to see in the picture. I remember that hinge plate being that way when I got and 3 others in the 10167 Brickmaster Kit (with Digital Designer CD) back in 2004, so I don't believe it was faded or a defect.
So I've long wondered, what's the deal here? Are the hinged plates made of different plastic, like the transparent pieces are?