Aurora Borealis or the northern lights aka. one of the most astonishing phenomena on the planet. 
Fun fact: the Finnish name for the phenomenon is "revontulet" which roughly translates as "the fox's lights". In times gone by, it was believed they were sparks produced when the fur of the elusive fire fox that roamed Lapland struck the ground or the trees. 

7 colours in total, in case you're interested in that sort of stuff.