Hey!

You should use terminfo instead of relying on the terminal name in TERM. This can provide you the number of colors supported. Otherwise, you are missing a lot of terminals supporting colors, like rxvt or screen. In Go, it seems https://github.com/xo/terminfo can do that. It also seems to handle some edge cases for terminals not using the correct TERM. It also handles the special case of true color which was not available through terminfo until recently and is able to fallback to COLORTERM (but it should be using Tc if available).