The rules sheet for the Portals found in Portal Kombat can be found
here. I'm not sure how to sum them up other than to say that if there are two or more Portals on the board of the same color, you can go through one Portal to one of the rooms on either side of one of the Portals of same color. You also can just treat the link as a standard hallway, rather than travel to another Portal. I think the graphic on page 2 describes things pretty well.