Why are these hinged plates slightly different colors?
Quorneng's answer is spot on, even bricks that were from the same set might have been made from differing batches of a particular color and slight variances will produce differing colors. Purple is the most notorious Lego color for having a great deal of variance between bricks, and red is not surprisingly, also inconsistent at times.

To add to the mix, Lego is changing its formulas over time to keep costs down, and to make the bricks more durable and consistent. Earlier ABS bricks where stiffer and more brittle than modern bricks. They have more of a tomato red color, and are easy to spot next to modern bricks.

UV damage can alter the color of bricks over time, although this can be fixed with oxyclean + h202.

Finally, I believe that Lego will internally recycle 'bad' batches of bricks, grinding up and melting down ABS from lots that don't pass QA. This probably also affects brick color. I had a friend who made the pilgrimage to Billund to see the main plant. He told me about how they have a display that shows the casting of bricks to visitors, and it re-uses plastic for the demonstration. I recall that he commented on how the plastic colors were slightly odd looking, presumably because it had been recast so often. This makes sense to me, but it is a second hand story, so don't cite me for your thesis...