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Glass doesn't like to be painted. Will scratch off and peel easily. Clean extremely well and spray paint would work best. If not an option , use a binding primer before you paint. Make sure it's really something you want to do before ruining the tile.
Hi Merri, here's a post from someone who did it. http://rosabeltrandesign.blogspot.com/2013/02/diy-painted-tile-backsplash.html
Read this blog https://www.addicted2decorating.com/
Kristi paints tiles all the time.
Hi Merri, here is the Hometalk post on painting glass tiles