I was think about numbers in console (iex) and tests. It’s hard to read bigger numbers.
Of course in tests we have colors for changes (thanks!), but its only for tests and only when comparing results by assert macro.
So here is my proposition:

Environment

• Elixir & Erlang versions (elixir -v):

Erlang/OTP 18 [erts-7.3] [source] [64-bit] [smp:4:4] [async-threads:10]

Elixir 1.5.0-dev (40c55f3)

Erlang is from portage. Elixir is compiled from source.

• Operating system:

Funtoo Linux (Gentoo based) stable (stage1 setup) with latest updates.

Test code:

defmodule MyApp do
  def number_a(), do: 100000
  # ...
  def number_b(), do: 1000000
end

defmodule MyAppTest do
  use ExUnit.Case

  doctest MyApp

  import MyApp

  test "numbers" do
    assert number_a() == number_b()
  end
end

Current behavior

iex(1)> num = 1_000_000
1000000
iex(2)> IO.puts num    
1000000
:ok
iex(3)> IO.inspect num
1000000
1000000

mix test

  1) test numbers (MyAppTest)
     test/my_app_test.exs:9
     Assertion with == failed
     code: a == b
     lhs:  1000000
     rhs:  100000
     stacktrace:
       test/my_app_test.exs:9: (test)



Finished in 0.1 seconds
2 tests, 1 failure

Expected behavior

iex(1)> num = 1_000_000
1_000_000
iex(2)> IO.puts num    
1_000_000
:ok
iex(3)> IO.inspect num
1_000_000
1_000_000

mix test

  1) test numbers (MyAppTest)
     test/my_app_test.exs:9
     Assertion with == failed
     code: a == b
     lhs:  1_000_000
     rhs:  100_000
     stacktrace:
       test/my_app_test.exs:9: (test)



Finished in 0.1 seconds
2 tests, 1 failure

What do you think about it?