Blue = Captured turfs (the ones you own) always appear in a turquoise blue (the same color as the nav bar). You'll see these in areas where competition is available and you are the one with the most points within your competition group.
Red = Rival turfs always appear in red. You'll see these where your top friend in that turf has the same number of points as you. These are vulnerable to being captured!
Grey = Lost turfs always appear in grey. You'll see these where one of your friends has more points in that turf than you do.
White = Scouted turfs appear in white with a grey border. You'll see there when you have friends who have points in turfs that you've never been to. These are fun to be able to see which of your friends have been to these turfs and how many points they have. Especially cool when your friends go traveling!