doubleofive said:

Why are movies not colored anymore, even movies that originally had color?

If you're going back to the original camera negative (o-neg) for your source (which has the most detail), you immediately lose all the color timing. To preserve the original color timing, you need the final interpositive print that was used to make the dupe negative that release prints were struck from. Problem there is dupe grain due to the fact that it's a couple generations away from the o-neg.

So, to get the low-grain, super-sharp look that the average consumer expects from HD material, you can either scan the IP and DNR the hell out of it, or you can scan the o-neg and color grade it again from scratch.