Is the commission received from the supplier permissible or impermissible, and what should be done if it is impermissible, especially since my colleague will continue to take the full commission whether I take my share or not?

You are not permitted to take a commission from your friend or the supplier. Your friend does not deserve a commission on the supplier's transactions in which he expended no effort; rather, he deserves it only for his initial endeavor to get your company to deal with him. Anything else from the transactions that occur between the supplier and your company, in which the intermediary had no involvement, entitles him to nothing. Rather, taking it would be by false means.