Why does connecting a Macbook Pro to a 2160p external monitor via Mini-DP result in a mess of green/purple colors?
First thing I'd try would be a different cable.

Apple has this to say about non-compliant Display Port cabling, though not specifically in response to your particular issue, but Google brought me there whilst on that search...


  Certain cables advertised as being VESA 1.1a compliant may contain a construction issue that may result in the DisplayPort power pins at either end of the cable shorting together. VESA 1.1a compliant cables must isolate the DisplayPort power pins at either end of the cable.
  To learn more about the VESA 1.1a DisplayPort Interoperability Guidelines, visit www.vesa.org