OK! Some progress. Colour rendering now sorted, thanks to these commands:
ln -sf /usr/lib/aarch64-linux-gnu/libGLESv2.so /usr/lib/chromium/libGLESv2.so
ln -sf /usr/lib/aarch64-linux-gnu/libEGL.so /usr/lib/chromium/libEGL.so
As noted here: [Solved] Color depth issue with Chromium Kiosk - #7 by locutusweb
Just the resolution issue to crack.