Can I ask what TV you have? The one here is an older Panasonic Viera.
The peculiarity with what I see, is that the Youview box menus (ie what's on now/next) come up in normal colour. It's only the main picture that's affected. It's also difficult to tell if it's the TV that's been changed or just the Youview box - as presumably by changing the Youview box from 1080p to 1080i (and then leave it it at 1080p), that sends some form of reset to the TV (and I don't have NowTV as an app directly on the TV to check)? I can't believe everyone with NowTV on Youview is getting the same issue (although there are a few reports of this on the web, there don't appear to be enough for a global problem).
I don't have the issue either.
BT box Pro, hdmi cable supplied with the box, TV is a Sony bravia 37" (can't remember the model) but its circa 2010 model. Having said that I use a Sony str-dh800 av receiver in between. I do note that on first power up of the stack, hdmi dances about for about three times. Then settles and its perfect.
I wonder if it's something to do with hdcp as I'm sure sky want that feed 100% locked down and protected from being re-streamed. Perhaps software is checking the handshake and that upsets certain TV models?
Also doing the resolution switch would definitely reset the handshake as the box will probe the tv's edid for supported resolutions...