Marc Whitten, Xbox Corporate VP Marc, shot out a Tweet in response to Kotaku's Stephen Totilo about their feature on Xbox One Achievements, and it looks like the color they're have will be completely dependent on you.
We've seen these colored Achievements before. At around the 11:17 mark on this recorded footage of Powerstar Golf on Microsoft Studios' livestream on Twitch, the Achievement appears in a deep cyan color. If you haven't seen the Xbox One Achievements yet, you can note the smooth transitions of the indication, with the name and value of the Achievement sliding in and out without any fuss.
Being the resident ninja at GameRevolution, my color will be black, or as Tom Haverford would say, Obsidian, Onyx, Midnght, Lost Soul, Rolling Blackout, Sleeping Panther, or Void by Armani.
What will your color be? You can say "default."