The most recent version of nix supports coloured build outputs even when doing remote builds by specifying the build machines with the ssh-ng:// protocol in the /etc/nix/machines file.
This in itself works great, however I also have configured a hydra instance on that particular machine. Hydra's parser of the machines file extracts 'ssh-ng://hostname' as the hostname, which results in connection failure. So there currently is no way to have both the coloured build logs on manually triggered builds and a hydra instance running on the same machine. (Not trying to achieve coloured hydra logs here, but that issue is kind of related: #148)