So far as I know, the pre-recorded discs (DVD ROM) are gold in colour while the mass market recordable discs (DVD R/RW) are purple in colour.
This is apparently because the former are created by physical pressing in the factory with moulds which creates "lands & pits" on the surface whereas the latter contain organic dye on the surface on which the laser acts during recording.
The 2 processes are different and hence the difference in construction / colour.
P.S. - Request techies to polish / supplement the above.