Roasting a turkey in a roaster oven
Since the roaster oven is so much smaller and has a fairly tight fitting lid, you are left with an environment that more steams than roasts in a nice dry heat.  Because of that, you will be left with a not so pretty bird.  Roasting it in the oven first might give you the color, but the slow moist cooking of a roaster oven will soften the skin to a less than pleasant texture.  Your best bet is to cook the turkey in the roaster oven first, THEN roast it in a high heat in the oven to brown it up and crisp up the skin.  

I would suggest doing the last half hour in the oven.  Lightly butter the skin and turn the oven to 400°F to finish it off.  The best part is, all the drippings will already be in the roaster, so you can pour that off and make the gravy while the turkey is finishing up.  And if you have a large enough roaster, you can set it to low, throw in some of your side dishes and use it as a little hot box to keep everything nice and toasty.