Weird date-related dynamic background colour behaviour

I have recently moved a photo blog from wordpress.com to self hosting. The wordpress.com site used theme Monotone. This is no longer available from www.remarpro.com, so I used theme Duotone there instead. Both these themes are supposed to provide a “dynamic background color based on the colors in your photos”. This should affect the overall background (to the left and right of the photo) and also the background where the blog title appears and the widget area (above and below the photo).

You can see this working in Monotone at
https://noticeboards.wordpress.com/2014/11/03/ffwrwm-at-caerleon/
and in Duotone at
https://noticeboards.artprop.org.uk/2014/11/04/an-empty-billboard-site-in-sao-paolo-brazil/

However I notice that although the dynamic background colours work well in Duotone for posts in 2014 and later, it does not for posts in 2013 and before. The main background is always white and the title background is always a mid grey. See this at
https://noticeboards.artprop.org.uk/2013/03/17/calle-valverde-madrid/
compared with the Monotone version at
https://noticeboards.wordpress.com/2013/03/17/calle-valverde-madrid/

Any ideas why this appears to be date-related? Or how to fix it?

I installed the blog with Duotone on my local computer as well and had the same problems.