Why is the victory/defeat screen sometimes in color and sometimes desaturated?

Question

Sometimes the victory/defeat screen has the game board behind it in full color:

Sometimes the game board is desaturated, almost but not quite grayscale:

(All screenshots are from playing a Tavern Brawl on the Android version of Hearthstone; the fourth is from a phone instead of a tablet hence the layout variation, but this doesn't seem to account for the difference.)

What's the logic behind this?

Disproven hypotheses:

• It can't be exclusively whether you won or lost; as illustrated above all four combinations are possible.
• It can't be exclusively "gray if you get a 10 gold reward", because the fourth screenshot contradicts that.
• It can't depend on the battlefield theme because all four above are the same one.
• It can't be "gray if you got any kind of reward", because I've gotten quest rewards with a colorful screen (in Casual) and been defeated with no rewards with a gray screen (in Arena).

Answer 1

I have conducted a small experiment. Over the last two weeks (I do play quite heavily) I have won 154 games.

54% of these I have had a colourless win-screen. The rest of these (46% have had coloured win screens).

There are no apparant factors which effect the colour..