Should you "rest" a roast? I recently made roast beef that cooked for ~ 8 hours. I took it out of the crockpot and let it sit for about 10 minutes. Then I sliced it into pieces and we ate it. It was pretty good. The leftovers were packed away. The odd thing was the next day at lunch, the meat, which seemed uniformly brown the day before, was now a nice pinkish color in the middle (keep in mind I had cut the entire thing, so the slice I'm eating is brown on the outside and pink on the inside, maybe 1/2 thick). It also seemed much more tender. 

What happened here? The leftover roast seemed much better than the meat from the night before. I am discounting the probability that the roast beef fairy came by and swapped out our leftovers with better ones. Should I have let the roast rest longer? What is the procedure for letting something cooked at such low temperatures rest anyways? It's not like a steak that was cooked at high temperature on a grill.
Let it rest anyway. Even at low temperatures, the juices will be flowing through the meat. Resting stops this process.

As for the strange colour-changing meat, I suspect you just weren't observant. Meat that has gone brown will not spontaneously go pink again.