Ah, rhubarb! One of spring's treats! Yes, you can pick partially green stalks. Supposedly (I've never grown Canadian Red) you're growing one of the sweetest red stemmed varieties. You can, however, pick stems when they're not fully red. I like to pick rhubarb when the stems are slender--that means they're less pithy or woody and cook down quicker. The more you pick, the more you will have.
If you see any flower shoots forming, remove those immediately to prolong the harvest season. As long as you keep those flowers from forming, you'll be able to keep harvesting.
Remember that rhubarb is a heavy feeder and loves a dressing of manure in late fall, if you can get any clean manure. Otherwise, a nice mulch of compost is great--in spring and again in midsummer if it has broken down, then again in fall. Really--their appetite is that big! Enjoy your rhubarb!!