I think its Red. Becz in her Red tour at New Jersey she siad that for her the clr Red is full of emotions and thats why its her fav.
Check the video in the link, might helpful: link
I have read in a few places that she does not have a favorite color :( I was searching around and came across the website taylorswiftweb.net that mentions all her favorites like food, books etc but not her favorite color !!