For the ganache you need to use oil based food colorings. I use powdered oil based food coloring from Russian brand
Kondi Pro. Their food colorings are great for ganache and chocolate, I really love their color palette. You can order from their website, they have worldwide delivery. I also recommend
Roxy&Rich and
ColorMill oil based food colorings, their food colorings dissolve in chocolate well. I cannot speak for other brands. A lot of brands claim that their food colorings work for chocolate, but they actually work only for compound chocolate that has substitute fat such as palm oil, and do not work for real chocolate that has cocoa butter in it.