The Interstitial Ad format is governed by the provider that you are using. I am not sure exactly whether or not AdColony (given your case) supports portrait ads also, but generally, you can change the parameters of the served ads via the dashboard on the provider's website.
While some providers do indeed let you change parameters regarding the way the ads are served to your users, it is generally not recommended to do so, as this might and in many cases will result in a weaker eCPM, since you are restricting some ad formats that at given times, might yield more money.