You can get different grades of canvas, sailcloth or whatever dyed to whatever color you want if a color already dyed is not available.
Your going to need a sailmaker or awning or street banner (might see if a banner company makes a stock size that would work for you) company to sew these for you or they will pull apart in seconds. You should probably have them sew some vents in to reduce the wind load on the posts you have in mind to hold it. I do not think your proposed 3' extension would pull your fence over. Don't take the power of the wind lightly though. You are proposing to add substantially to the live loads against the fence. And you will need some strong posts/brackets for these things.
You sure adding some plant screening materials to the landscape would not work out better?
Certainly where I am adding 3' of canvas to a fence top would be regarded an illegal extension of the allowable fence height. I would need a variance and permit. A series of long flopping banners placed close together with poles on my side of the fence might not be. Hint. Hint.