If you hadn’t tried an external display I would have said you must be taking some good stuff to see these chroma effects!
Have you tried checking the display settings under the System Preference’s?
As your external is working correctly it does narrow things down to the internal display logic. Unlike older models which used iDP (also known as eDP) which is an internal version of DisplayPort, this system uses a direct feed from the GPU logic to the T-CON board within the main case then the ribbon cables interconnect to the LCD within the display. Here’s a view of the connections on the T-CON board.
The T-CON and the display are all within the same assembly. There is no way to fix this other than replacing the full assembly. Is your system covered under AppleCare or AppleCare + If it is then you’ll need to visit an Apple Store or authorized service center. If not its not going to be cheap!