Thanks for the reply @turnermm!
So, using the example on the Geshi documentation, I can put:
$geshi->add_keyword_group(10, 'color: #600000;', false, array('myfunc_1', 'myfunc_2', 'myfunc_3'));
in a file named something like:
I'll give it a shot and see what happens.
EDIT: Not working yet. I'll try again in the morning.