[racket] [help] : Syntax coloration

Hello,
I am a french student and I have to do a project for my studies...
The project is a compilator from a lambda language to racket.

I use the racket/gui and i defined an editor-canvas% to write in my
lambda language. I would like implement the syntax coloring for my
language.
To do this, I re defined the on-char method ( define/override ) but,
when i write now on my keyboard, the text doesn't appear in my
editor-canvas%....
Is it possible to just add at the end of the function on-char (without
changing the on-char), a new function called "color-key" (for example)
(previously defined by me) to coloring the text just insered ?

Thank you very much
Best regards