Do not go to your local auto parts store and just see if you can find a Toyota paint that appears to match. It won't.
Open your driver's side door. Take a look on the frame of the car where the door meets the frame. There you will find a panel with all sorts of information about your car, including the code number indicating which paint was used on your Yaris. The code is found on the lower left hand side of the panel. That code begins C/TR: and is followed by a three digit/letter code followed by a slash and another code you don't have to worry about. The paint code number in this case is 8T1: which according to Automotive Touch Up (http://www.automotivetouchup.com/touch-up-paint/toyota) is known as "Bayou Blue Metallic."
After adventuring in local automotive shops, while I believe this color to be common as dirt in reality, it is nowhere to be found on the shelves of the standard stores. You'll likely need to go online to some reliable source to find your color. Good luck. Happy touch ups!
To solve other Yaris maintenance mysteries, see: http://jsbrookspresents.blogspot.com/2011/05/resetting-clock-on-your-toyota-yaris.html, http://jsbrookspresents.blogspot.com/2010/08/turning-off-2009-toyota-yaris.html, and http://jsbrookspresents.blogspot.com/2013/04/toyota-yaris-finding-your-jack-and-tire.html