I see what you mean.
Just one thing, "aqua color" is called "cyan".
1) The floor and ceiling of the control sector of the FOF should be the same ! For example : FLOOR : 1000 CEILING : 1000
2) If it does not work, then do this manipulation.
Switch to linedef mode. Right click on linedef that controls the FOF. Go to the tab where you can choose the texture of the wall. If I remember correctly, you need to type in the second square of the first line, write #255
I think it's this if my memory doesn't plays me tricks.