Gentrification - and its reverse

The Economist links to an interesting map showing which bits of London have got more gentrified and which have moved downmarket.

I found three things interesting. (1) Why is red upmarket and blue downmarket? Red is hot and strong, while blue is cold and sad? Maybe. But gentrification has political consequences and red and blue have political meanings. I found it a strange decision.

(2) There's a lot of blue. Much more than there is red. And there looks to be plenty of blue off the map too. Is Greater London as a whole moving downmarket? Is the UK?

(3) Brixton, almost a synonym for gentrification (even the Economist mentions its burgers - and the Economist is also serious about burgers), is not being gentrified. Just look at those blue patches on the map.