Currently is not possible to use ANSI escapes in prompt.

prompt = function(){
  csi = String.fromCharCode(0x1b) + '[';
  reset = csi + '0m';
  return '{0}db{1}'.format(csi + '44m', reset);
}

Will produce prompt like:

[44mdb[0m

I'm not sure if it's desired behavior for shell prompt. It would be nice if users could use color for shell prompt cusomization