You can buy FDA approved dyes that are concentrated in gourmet food stores and some large craft shops likely on eBay also. They would be the concentrated dyes used by manafactures without any flavorings, sugars etc like used Koolaid.
eBay sell:
http://cgi.ebay.com/New-Wilton-12-IC...item3353baaf28
The blue coloration that comes natural to some of the mainly indica crosses is typically caused by night time temperature drops of usually 20 degrees or more. Sometimes more drop isuch as if the day time temps are higher than around 80 degrees.