Subject: Why do electrical arcs look so different?

I have recently been experimenting with large arc and sparks. I have two 
transformers (and accompanying circuits) that step up normal 240v household 
voltage one has an output of  30 000v to 32 000v and and the other has an 
output of about 35 000v to 40 000v. after testing the arcs of both these 
transformers through air I found that the more powerful one produces a larger, 
jagged, purple arc whereas the smaller one produces a soft, blue and orange 
stream of plasma. Why is there such a drastic difference? What could cause 
this? 

Re: Why do electrical arcs look so different?