I'm using git 2.14.2 and have seen this issue for more than a month on my mac. I have not installed any artificial tcl/tk (which also does not resolve the problem). I'm now using High Sierra but seen that problem also on macOS Sierra before. The setup diff program is opendiff (the default one here).
Does somebody know how to get the diff colors back in gitk?
I only see e.g. this codes [1;33m
and not the actual diff colors anymore