I am planning to travel to Toronto in March 2018, but I don’t know if I can apply for an Electronic Travel Authorization (eTA) with my U.S Green Card expired and the receipt I-751, which extend my Green Card up to September 2018, or if I have to apply for a visa since the Canadian law requires visa for Colombians.
I am not sure. This FAQ says that for eTA you need a valid green card, or an I-551 stamp on your passport. It doesn't mention an expired conditional card plus extension letter as acceptable, though the answer might just be incomplete.
People applying for Removal of Conditions don't usually bother to get I-551 stamps, unless they lose the extension letter or the one year is close to ending, because otherwise the expired green card plus extension letter is valid for all official US purposes. Some offices might not even give I-551 stamps to people who already have an extension letter that's not close to expiring, unless you have a good reason (though needing it for Canadian eTA would be a good reason). So it might be safer to go to the local USCIS office and get an I-551 stamp.
I looked on the eTA application itself, and it doesn't ask whether you have a card or I-551 stamp specifically; it just asks for your A-number and "date of expiry". Presumably people with an I-551 stamp would put the expiration date of the 1-year stamp. If you want to try using just the expired green card and extension letter without getting an I-551 stamp, I guess the most correct thing to do would be to put 1 year beyond the expiration of the green card as the date of expiry.