Apple TV Changing Colour Resolution

I have a problem when streaming Apple TV programmes on my TV (I’m doing it via a Fire Stick). The display changes intermittently between full HD colours and muted copies throughout streaming. It’s exactly the same problem as discussed in another thread here, and they also note this only happens with Apple TV and not any other steaming service to their TV:

Color changes while watching Apple TV+ pr… - Apple Community

It seems a lot of people have this issue but the thread was closed immediately asking people to look at iPhone settings which is just about downloading programmes on WiFi or mobile data. Not even relevant. 

Can anyone help? Googling and ChatGPT are not coming to the rescue here either.