I have the exact same issue. I could be watching Prime and once back on any channel the colours are severely bright.
Need to go through process or power off 360 box, unplugging hdmi cable, replacing cables etc
Checked settings numerous occasions and all fine. If switching to inbuilt free view on tv the picture is always perfect.
Would agree with vast majority that it only happened once the box was updated to the 360 software.
Still no fix and nothing I do seems to resolve it permanently.