Here’s how I see the Mane 6 in terms of colors:
 
Applejack: Mono-Green. Green is the color of simplicity, and doesn’t care for many of the temptations and desires that the other colors feel. Given her status as the Bearer of Honesty her down-to-earth personality, it’s a safe bet that Applejack would be green.
 
Fluttershy: Green/White. The way she cares for animals and treats everyone with care show a strong respect for life in general and a desire for peace, embodying both White’s desire for peace and Green’s desire to let life flourish.
 
Pinkie Pie: Mono-Red. She’s impulsive, and above everything else wants to make sure that everyone has fun. Passion and emotional freedom are Red’s ideals, and going through great lengths to have fun embodies that.
 
Rainbow Dash: Red/White. A free spirit who is very passionate about following her dreams, but at the same time shows great devotion to her friends. While loyalty is one of White’s core values, pure White shows loyalty out of a sense of duty and obligation. Rainbow Dash isn’t the Bearer of Loyalty out of respect, she’s loyal to those she feels she should be, regardless of law and authority. She shows White’s dedication, but she does so by letting her emotions guide her as Red would.
 
Rarity: Blue/White. Her dedication to her craft is characteristic of Blue, as is her desire for perfection. Generosity is characteristic of both Blue and White, as Blue wants to improve the world with its work, and White wishes to benefit the community as a whole.
 
Twilight Sparkle: Mono-Blue. She’s a dedicated scholar, often crossing over into obsession, and the desire to learn is Blue’s strongest trait. The episode “Lesson Zero” also shows how she can embody some of Blue’s negative traits: her desire for perfection will sometimes drive her to do morally questionable things.

Laughter can easily be black. Laughter doesn’t have to be nice. It can be cold, villanous, maniacal…

Rainbow Dash - Red  
Rarity - Blue  
Fluttershy - White  
Applejack - Green  
Pinkie Pie - Black?  
Maybe Multicolor/artifact somwhere