Thanks for the feedback.
The main reason that such a colouring method is not supported is because of the sheer amount of memory it would require, ie. twice the map size in bytes.
In ECM mode you automatically get four different (background) colour versions of each of the 64 chars, but yes if you use "per char" colouring in any display mode you would potentially need to duplicate chars to get a different (foreground) colour version.
I will have a think about the problem and put it on the todo/maybe list.