Go to the end of the walkway. If you look up and to the right, you'll see the exit. Down below, there is an enclosed area where conversion gel is pouring from a tube.
Your first task is to get some conversion gel to paint an area outside of the enclosed area. You can do this simply by putting a portal under the spot where the gel is pouring, then make another portal on the floor nearby. The gel's momentum will carry it over the enclosure wall to the floor outside. You can coat every nearby surface this way if you want, but you don't have to.
If you're not in the enclosed area yet, drop down into there. Look through one of the metal walls at the conversion gel that you put outside of the enclosed area. Put a portal there, then a portal of the other color where you can reach (or just drop through the portal that the gel is falling into if it's still there).
There are two big columns next to the enclosed area, with numbers painted on them higher up. Make sure that you have a portal under the tube pouring the conversion gel. The next step is to put conversion gel between the two aforementionedcolumns. That way, you can have conversion gel pouring out of a portal on one column, which will coat the other column with conversion gel. Put a portal of the same color on the new conversion gel as high up as you can put it. You can continue doing this, alternating and raising the location of the portal, to coat the entire columns in the middle.
Eventually the gel will be high up enough that you can use portals to go up to the walkway high up on one of the two columns. Go up there. Now look to the left. There is a low metal platform to the left of the enclosed area. You need to go onto that platform. There is an angled wall next to the column that you are on. Use portals to coat the angled wall with conversion gel, then put a portal on the angled wall. Go to the other side of your column and put a portal on the floor below the open part of the walkway. Drop into it and you'll go out of the angled wall and onto the metal platform. (You can also just put a portal on the floor near the metal platform and fall into a portal from the column, but it's not as easy.)
Go up the stairs and go through the exit door.