Differently coloured door insides - this sort of thing was only ever seen towards the end of their life, after the trains had started being replaced.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47739225@N08">citytransportinfo</a>
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/citytransportinfo/50992617521/">C-Different-Colour-Door-Insides-P1310090</a>