It goes red-black-white-red. And since temperatures rarely jumps 5°c without gradient, the map will have black and gray area surrounding any white that represents +13, and white will surround and red that represents +18°.

White = -8, 0, and 13.5-ish

I've just realised that this is saying that the UK is warmer than France. Because despite the term being white hot it's actually decided that red is hotter than white. Why didn't they just go from white being cold to red been hot?