To change the color/texture of the inside wall sides, you should create rooms and modify those rooms, because their modification dialog box allows to set the color of the wall sides touching the room (see More color options for wall section here).
To change the color/texture of the outside wall sides, you should select the walls that are not oriented like the other ones and choose Plan > Reverse walls direction. Then, select walls you want and set the color/texture of the outside.