check all the Court.ini in the Modding SDK ; MaterialIn & MaterialOut in the section [Data] determine the surface colors.
Eventually, I'll do a system to allow adding new surfaces in 3D.NNUE
Did you read the "2D Court" section here : topic11-31376.php
? Especially the "/!\ Note for TE2013 2D Courts " ; I guess it's your issue.
Else, is it on grass ? If so, you have to put an empty Court_Used.jpg (or just duplicate your Court.jpg).