Colours on a Microbee

[someone]

How do I set the 256TC into 64 column mode?

Identically to any other microbee - by reprogramming the CRTC.
The best place to grab the default CRTC register values is from the MWB ROMs.
The standard initialisation routine configures 16 registers.
Also note that the 13.5MHz dot clock microbees use some different values than the 12MHz ones.