There is some debate amongst collectors that believe some of the aprons were released in purple, however it is primarily believed all aprons began life as blue and the purple is caused by degradation of the color dye used in the material on some ugnaughts. There are several factories that made them and some used a different material that has stayed blue. You can read more about it here: http://www.imperialgunneryforum.com/t4763-the-tig-fotw-thread-ugnaught
The fact that there are pictures of the tunics changing color from blue to purple is, in my opinion, enough for me to conclude that blue was the intended release color and purple is just fading.
I have caviar dreams and McDonald's pockets