Purple for royalty! 
Behold, my long flowing purple robes, with little white tufts, and my golden scepter and my globe with a cross atop it! 
Okay, I overdid it.  

 yeah, that one just hit me just now thinking it further

purple is the most energetic wavelength of visible light though, also...

oh and plants are green because both red and blue are more energetic than green

there are a few enzymes that need green light and they thus are purple 

so there's another metaphor in there too... purple also indicates a material that is reactive (derives energy) from green