How colours fade

[figheadair]

In all the Medieval and Gothic-period tapestries I have seen the greens and blues appear the least faded and red is often completely disappeared to off-white or yellow.

The red wold have faded because it would have been from madder or the like. A good insect red is pretty well indistructable too. Cochineal was the preferred red dye of choice even in the Highland by the early 1700s.

if the blue is from woad then it will never fade, the fabric will disintigrate before the blue colour leaves, a good example of this is the bayeux tapestry the blues are rich and deep after nearly 1000 years but the reds have muted into oranges and yellows even though it's been well cared for

Quite true although the blue would have been true indigo by the mid-1700 as opposed to woad. Same comment about the red as above.