Blue is just the natural lighting playing tricks with you, and it looks really good, and I see you got a ton of snow. I made a cold frame like that one year many many years ago, but I found I developed fungas as a result of the greenhouse plastic. The plastic filters the suns strongest spectrum and you may get while fungas growing everywhere inside the pond, even on the fish. After I took the cold frame off the fungas quickly went away by itself, as the sun burnt it up naturally. No fish were injured by the white fungas and it too went away when the sun shone on the pond. Just keep an eye out inside the cold frame I case it happens to you.
It has been over 20 years now since I had covered the pond that one winter, and have not seen the fungus again. I do heat the water when temperatures fall below minis 10 C. I used to work at a large greenhouse that had plastic frames on one side of the lot. The frames that had glass, we never had trouble with mold and fungas, but in the plastic frames we always were using a fungicide to pervent the mold and fungas.
I really enjoyed your beautiful winter wonderland pictures!