Remember the Mod Podge + food coloring technique for painting clear glass? That was cool and all, but I spotted another method using Vitrail glass paint and acetone (nail polish remover) that allows the bottles and jars to be used as actual vases, as the finish is waterproof! This treatment provides for more depth of color as well. Head on over to My Design Folder for all the details. Vitrail glass paints start at around $8-$10 and come in lots of colors including (but not limited to): Red, Cobalt Blue, and Sun Yellow.
Update: We've tested two new techniques for dyeing glass jars. Check it out here!
Buy Or DIY?: You can also buy colored jars on Etsy for $15-$30 to save yourself some time.