now for flaring the vent..
first you start off with the port the regular way.....
then you draw a square from the corner of the vent exit to the width that you want to make it , say you want a 3" flare, make the square 3"x3"
now draw a circle to from the edge of the square to the edge of the outside of the box.
next you start from the cender again but go to the inside edge.
now erase the edge of the square
erase the extra part of the circle.........
now you have a finished flared vent....
now you make a line on either of the ends.....and make the floor of the box solid again....
then pull it up and your good to go.
and if you want the lines to go away where the flare is, hold control and use the erase tool. it will leave the part there and erase the line.
As for adding color and textures go the the paint bucket, and go to carpets for carpet or colors for color. ( pretty self explanitory )
and if you dont want to have to color each piece the same color, courser to select the whole project and then paint it as one object