System Information
Debian 8, nVidia GTX970

Blender Version
2.74

Short description of error
The matte created by the chroma key node is far away from expectations. As a test case I used a HSV color wheel. The expectation is that the chroma key filters out a sector of this color circle. Instead the result is all sorts of strange curves (depending on key color) through the color circle. E.g. for green it is a line through the center also including the inverse color of green. For other colors it is some U shape in the color space.

Exact steps for others to reproduce the error
Create am image node with a HSV color wheel, feed this into a chroma key node and look at the result.

Sample blender file:
http://www.pasteall.org/blend/37548

Screenshot
http://www.pasteall.org/pic/91895

Best regards,

Michael