Running highlight-regexp in elisp function - not working

[Angus Comber]

Hi

I want to run this elisp code to highlight various sections of text in a
particular type of log file.

I tried:

(defun abc-log ()
  "abc log file highlighting"
  (interactive)
  (goto-char (point-min))
  (highlight-regexp "message Event.*" hi-yellow)
)

But when I run I get error:

Symbol's value as variable is void: hi-yellow

And it doesn't highlight.

I just want to be able to highlight-regexp on specific text and use
hi-yellow.  What am I doing wrong?

Angus