If you keep your trousers or jeans on no-one is going to know what colour your pants are so if you want to wear black ones, I don't see any problem.
Regarding what else to wear, whatever you find most comfortable and that doesn't "jar" with the other students and staff would seem to be a good starting point. Good advice has already been given as regards best fabrics for safety. Instructors are normally more interested in whether you've had regular showers than in what you wear (within reason).