Suppress stderr while running Mix Test in Elixir

I'm working with several external processes in my application. During tests the stderr output of several of these is output inline with my test messages and results. I can do this:

mix test --trace 2> error.log

However, when I do, I lose all my lovely colors. Also some Elixir errors still appear, though not all (which is fine for me).

Is there a better way to suppress the errors of the external programs without affecting the mix output? Is it even a good idea?

Or should my tests not actually interact with the real command-line utilities? I ask because at that point I'm honestly no longer clear on what I'm testing.