"How how long do I have to get title and mount rewards for the achievements associated with Wrath raids and heroics? Is it until patch 3.1 when Ulduar gear will make them easier? Until the next expansion? Some time in between?"
There has been no official information. However we can look to the past to predict the future. Blizzard removed the ability to get the "Hand of A'dal" title in patch 3.0.2, along with the bear mount from Zul'Aman. I would expect them to do the same to all raiding titles and rewards with the 4.0.2 patch that will come about right before the release of the next expansion. I would expect you'll always be able to get the achievements, however. It'll just be the rewards that will change.
However with that said, the focus on achievements to delineate hard mode encounters and progression tactics is something new. Blizzard might have a different game plan in store for us, and they might just allow the titles and mounts to be achieved indefinitely. That would be rather cool I think, since plowing through Sarth 3D at level 100 will just make me smile, much like 2-manning Onyxia did back in BC (my resto shaman friend and my prot warrior took 45 minutes to do it, was great fun).
"Why are there four color sets for T8 armor? 10 man, 25 man, and then what? Are they just re-coloring for PvP sets again?"
The second part of your question first. No, they are not re-coloring armor for PvP sets. Blizzard has said numerous times that one of the "mistakes" of BC was to make the PvP sets look the same as the raiding sets, and that they've learned from their mistakes and will make the sets visually different. There is no indication that they're going back on this.
Now why the different colors? As you said two of them will be for 10-man and 25-man sets, but that leaves two more. There's a bunch of possible answers, but all of it will be speculation more or less. Elizabeth Harper, our Editor-in-Chief, suggests that this change might be a prelude to armor dyes, something that would be very cool. Alex Ziebart points out that Blizzard has been doing a ton of re-coloring lately on many models, making it easier to add some variety into raid content. I tend to agree with Alex, and think that the re-coloring is just there for NPCs / mobs to have.