tolkien - Why is the Depiction of the Last Alliance in Rivendell faded out in Lord of the Rings but in Color in The Hobbit?

It's just the light

Here's a better image of the mural, taken from the Return of the King extended edition special features¹, where you can clearly see it in full colour:

Consider this still from Fellowship of the Ring, when Boromir is looking at the painting:

It's faint, but there is colour there; there's some yellow in the bottom-right corner.

Compare the above image to this one, from An Unexpected Journey:

In particular, consider the right-hand edge of the painting, where the shadows are covering it.

Both paintings are clearly in colour, but in Fellowship of the Ring we only see it under low light, making it appear more blueish.

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¹ A gallery of the Rivendell murals was one of the exclusive online content features on the EE DVD. The online content is now defunct, but Magpie, a user on TheOneRing.net forums, archived the murals on Picasa; may Eru bless you, Magpie