There are three red ties in Series 6. To really nail down the differences you have to take a close look over a LOT of images because as Harold points out image quality is not always what we would hope for in the episodes. These are just a few shots to give an idea of what I'm on about. And it also helps to think in production order rather than the more usual order of episodes we are used to.
Red1
Top image in this montage is from the early Series 6 publicity photos so I guess was taken a little after A Christmas Carol was made, it looks to be the same tie from that episode. Certainly these publicity photos were taken before filming on Series 6 started because the jacket hasn't had the elbow patches put on yet.
The first episode of Series 6 to be filmed was Night Terrors, middle row shows two shots of the tie in that episode. Next episode made was The Doctor's Wife, bottom left shot is that tie. Important thing to notice on these photos is that the detail of the tie varies from episode to episode because it is self-tie so the knot and folds are different each time it is tied. Bottom right shot shows it untied in a brief appearance in the next episode made,The Impossible Astronaut.
After the Night Terrors / Doctor's Wife recording block used Red1, the next two blocks [Impossible Astronaut / Day of the Moon and Rebel Flesh / Almost People] used the blue ties. Then a new tie is introduced.
Red2
The next block was A Good Man Goes to War and Curse of the Black Spot and both use the same new red tie, a pre-tied kind. It can be seen it is pre-tied because it always looks the same between the two episodes [top image is Curse]. Very similar to Red1 but it seems maybe a touch smaller and the detail in the pattern is a little different. The Red1 pattern seems to run more vertical while Red2 has a horizontal position. These shots from A Good Man Goes to War show the horizontal direction.
Red2 only appears in A Good Man Goes to War and Curse of the Black Spot. I guess that after spending a lot of time on Curse soaking wet the tie didn't recover too well and was ditched. Next episode made was The God Complex with a blue tie and then the replacement for Red2 arrives.
Red3
The new red tie is pretty much exactly the same as Red2. But because it is also pre-tied it has a unique shape to the knot and folds and can be matched from episode to episode. The Girl Who Waited and Closing Time were made next and both use Red3. Then it was Let's Kill Hitler which uses a blue early in the episode, then switches to white, then switches to red and it is Red3. Wedding of River Song next, with Red3 again and also the blue ties. After Series 6 The Doctor The Widow and The Wardrobe also used Red3.
The episodes being shown in a different order jumbles all the ties up but looking at it in production order there is a logical flow, Red1 is replaced by Red2 and that is replaced by Red3. So in Series 6 the red self-tie only appears in The Doctor's Wife and Night Terrors. The two pre-tied reds appear in Curse of the Black Spot, A Good Man Goes to War, Let's Kill Hitler, The Girl Who Waited, Closing Time and Wedding of River Song [plus The Doctor the Widow and the Wardrobe].
