To change the colour of the TODO items in the text editor, change the environment settings using Visual Studio (Tools -> Options -> Environment -> Fonts and Colors)
Then scroll to the item in the “Display items” list which in this case is “Resharper TODO item”
UPDATE: this applies to Resharper 7.1 and 8.2 upwards as pointed out in this comment
For Resharper 8 < 8.2 please see the solution below by @Soeholm