I had connected my laptop before and it worked fine so I knew that my TV and the HDMI cable were working correctly. I had recently updated the drivers for my NVIDIA graphics card so I went into the NVIDA Control Panel.
I went to Display | Adjust desktop color settings and played with the various settings:
When I moved the Digital Vibrance selector I could see the color turn from Gray scale to Color.
A setting of 50% for Digital Vibrance seemed to produce correct colors: