The curtains' colour had faded in the sun. [Wrong]

GRAMMAR INCORRECT TO CORRECT
Incorrect: The curtains' colour had faded in the sun.
Correct: The colour of the curtains had faded in the sun.

We do not normally add - 's/-s' to an inanimate noun [curtains']. Instead, we use an of-phrase 'the end of the road', 'the outskirts of the town', 'the back of the door'.

Note however that time expressions usually take -'s/-s': six weeks' work, a month's salary in advance.