There is a column in the distance that has a white wall on it. Put a portal there. Now put a portal of the other color under the orange gel tube. Push the red button. This will put a streak of orange gel on the floor near you.

Part of the ceiling has an orange mark on it. Put a portal there. Now put a portal under the blue gel tube. Press the button to turn it on. This will put blue gel at the end of the path of orange gel.

Now go to the wall where the orange gel starts, and run toward the blue gel. You should be bounced over to the column in the distance. If you go around the column, you can see the exit door on another distant column.

On the back side of the column that you're next to, there is a white wall far up there. Put a portal there, then put a portal of the other color on a white wall on the column that you can walk into. Go through.

From the top of the column, go and put a portal of the other color on a wall in the main testing area. Go through the portal near you. Now make sure to leave the portal on the high part of the column that you just walked through, and make a portal of the other color on the side of the column that you can now see from the main testing area.

Run along the orange gel toward the blue gel like before. you should go through the portal on the base of the column and come out on the other side of the column, farther up. This will throw you over to the elevator. Go in.