I had never considered before today how to change the colour of some text. I had messed with applying faces to fonts before when making major modes, but I had never looked into styling a specific string. It turns out to be easy:

(propertize STRING &rest PROPERTIES)

Return a copy of STRING with text properties added.
First argument is the string to copy.
Remaining arguments form a sequence of PROPERTY VALUE pairs for text properties to add to the result.

We could leave mak::last-command-status alone, and then advise the function to manipulate its output, but that will inevitably confuse me some day when I am debugging something. So we'll just add a simple propertize call to make the exit code of a command even more visible:

(defun mak::last-command-status ()
  (let ((c eshell-last-command-status))
    (when (and c (not (eq 0 c)))
      (propertize (format "?%d " c) 'face '(:foreground "red")))))