I have the Nissan Titan rims on my truck. They will not fit without some alteration. Alot of six lug wheels will fit on our trucks if you get the center bore machined out. I paid $150 for a local machine shop to bore these rims out for me.
Lug pattern is 6x5.5" or 6x139.7mm, you need a hub bore diameter larger than 100.1mm, and for offset it depends on what you're doing, how wide the wheels are and how much you plan on dropping it. For a 20x8.5 a +25mm or higher is recommended if you plan on dropping a lot, you can get away with something like a +15 or +18 but they'll be even with the fenders or stick out a little bit depending on if you have flares and what kind of flares.
Look for my thread on the American Racing Trench in this forum. They're offered in an all black 20x8.5 +30 offset specifically for the Colorado/H3/Canyon.
In that thread I posted there's an eBay link to someone selling the 17's. You might email them and ask if they can get the 20's. Other than that any AR dealer should be able to get them for you. I would guess pricing will be similar to what the RTS's sell for, so whats that ~$800 a set? :shrug: