I have white reflector material  for an old Bogen Lightform panel. The material has turned a dingy looking yellow. Can this be washed and, if so, how and what will happen in regards to color temperature?

If you wash it the reflective coating will disintegrate. You might as well buy a replace meant at calumet. They are on sale.

A local fabric store might stock that textile/material.  A salesperson can assist you in making a replica.