Travelling one week from Montreal to Toronto in the week before Halloween. I read there is a thing called "Indian summer" with wonderful colors through the country. But friends told me national parks will most likely to be closed (why?). Can you give me a rule of thumb when and where on the route you will likely to have open national parks during that time?
I think your friends are rather misinformed.
I am unaware of parks closing at any point during the year apart from the occasional holiday or similar occurrence. For example, the Oka national park is open all year round as can be seen here:
Parc national d'Oka is open year-round from 8 a.m. until sunset. Source
Basically, visit the information page of the parks you want to visit beforehand to confirm that it will indeed be open (they sometimes close for special occasions, but these are rare) and you will be fine. There is no reason for the national parks to be closed during the color changes.
Do keep in mind that, as Michael Seifert mentioned, the colors do not always coincide with the Indian summer.
As for where you can visit, I recommend you visit Oka national park, but also Mont-Saint-Hilaire and Mont-Saint-Bruno. The first is not a national park, but both are nearby Montreal (about 30 minutes by car) and feature some of the most beautiful scenery in the area.
Personally, I prefer Mont-Saint-Hilaire as it is taller and features more rugged terrain, but Mont-Saint-Bruno is perfect for a relaxed hike.
Hope you have a beautiful trip!
In Canada, many parks have a summer season. For example, the Mont-Tremblant provincial park in Quebec, where these color changes can be seen has its facilities mostly open during the high season (July and August) but most facilities open from mid-May to mid-October.
In general, in Eastern Canada, the summer is considered starting on Victoria Day weekend, around the 24th of May, and ending on the Thanksgiving weekend, the second weekend of October. You can consider all parks will be open during that time. "Open" just applies to facilities though. You can probably enter parks out of these times, just don't expect most facilities to be open, and the temperature might be low.
I have been to both the Mont-Tremblant and the Algonquin park around September/October, both are good to take advantage of this "Indian Summer". On Flickr, you can see many pictures of around the last week of October (in 2015) showing you will have nicely colored leaves.