Are you using fluro paint or a transparent paint only visible under UV light?
Either way I would avoid white and light colours for a surface as they reflect too much UV light to give a good contrast.
The transparent inks/paints (and to a lesser extent the fluro) show up badly on texture surfaces such as fabrics, I would also avoid any shiny surface as they reflect too much from the light source.
Go for something non-pourous, unpatterened with an even colour and with a matt smooth surface like vinyl paint.
Good luck.
They are traditionally secretive about techniques, but might be helpful about something as simple as paints and backgrounds, so try contacting any of the black light theatres in Prague.