You will need to document the original color.
Any paperwork for the car showing the original order info for the 999 color choice?
Do you have the broadcast sheet showing 999?
The fender tag looks to be original but the many layers of different paint obscure details to confirm it.
With 25 colors available in the normal 1970 Cuda color choices and two more added in the Spring of 1970 (FM3 - Moulin Rouge & FJ6 - Sassy Grass), it is rare to have a 999 custom color.
BTW, it appears to be a dark red and 1970 ER6 - Scorch Red was similar to your picture.
Do you have more pictures of the car and the trunk or under carpet areas showing the original custom color?