Game View allows you preview your map as it if was in game. If you select Game View (or press G on your keyboard) it will turn on the post processing and depending how you setup your PP it could appear dull.
Is this your map or one of the pre-existing ones?
I certainly noticed that if you've got an Uber Post Process in your Post Process Chain, it makes things dull. I removed it from my chain and I got vibrant white back. But then you're missing bloom etc.
I played around with settings for ages trying to get back to white instead of gray, to no avail. It's no issue to me personally, as I don't need bloom and wasn't using anything else, but that's just what I noticed.