Perhaps Pravana Violet.
I thought diluted down Blue Velvet at first glance, but the colour in the latest batch seems to be less blurple than that.
Maybe Adore's African Violet?
This is violet gem in my hair, it's quite close! have you bought the extensions already? or are you just wanting hair that's that colour? its loads easier to buy human hair and dye it but it is a bit more expensive.
http://www.hairdyeforum.com/index.php?topic=9551.0
eta - you'd probably have to dilute it a bit though as it is dark at first
I would go with diluted blue velvet. Dyed my bf hair with it last night and definitely looks like a darker version. It looks blue in some lights and more purple in others.