I had heard that buttercream darkens after settling but my butercream (violet and teal) both colors lightened after a few hours. 

Also airbrushed colors also lightened up after a while!!! What can i do to fix it. Thanks

Did you leave it out in sunlight? 

Those color especially, tend to fade when exposed to light.......sunlight or florescent.  

I didn't leave it in sunlight!! So surprised as to why it happened twice

The buttercream I have made ahead of time in past especially red always darkened up by the 2nd or 3rd day sitting in refrigerator. Don't understand why this happened!! Hopefully, the experts here will have the answer as to what to do. Good luck!!