Blue and orange gels work on bridges so why not conversion gel? In this map you can actually place portals on white-painted hard-light surfaces! It also offers a nice little medium-difficulty puzzle and a hammer-edditor-made visual design.
Just to be clear: I'm not a programmer to know how to make bridges always portalable when they are painted white. I've just put invisible portalable surfaces next to the bridges, which appear when they get painted so it only looks like they're portalable.
Because of a bug which I won't explain here I made the bridges be portalable only on the side where the player paints them. I know that if you paint a bridge blue on one side you are able bounce on both sides of it but...this is how things are 
I HIGHLY SUGGEST NOT TO USE QUICKSAVE OR QUICKLOAD (F6 and F7)!!! There's a bug which removes the paint from an already painted bridge when the player respawns.
