Why The 1959 Stock Seat Fabric Colour Change?

There were actually three different designs to be found on the Northern Line towards the end.

The green/blue/black was the earliest, which I presume dated back to around the time the trains arrived on the Northern Line (1970s). This could also be found on 1972MkII, 1973 and C stocks IIRC.

Somewhere around the late 1980s/early 1990s the pink/black design started to be used, which was specific to the Northern Line.

Originally the interiors were painted blue, however around the same time experiments were made with different internal colour schemes, with the cream eventually being settled upon. Only a proportion of trains received either the new moquette and/or the new paint scheme.

Meanwhile there were the handful of 1962 stock trains transferred from the Central Line, almost all of which retained the Central Line silver paint scheme with red/grey/black moquette. Compared to the 59 stock I always found these trains seemed in a rather better state internally.

On top of this, you can add the 'heritage' train which had its own moquette along with a green paint scheme and the prototype refurbishment train which IIRC had the 'Northern Line' moquette described above. The 72MkI stock had the red/grey/black moquette with blue interior painting (like the car preserved at Acton), and for a time there were up to 3x refurbished 72MkI stock trains too.