Polyester should be easy to tint, even food coloring should works as the water has minimal effect on polyester...
You can also take some powdered RIT dye and dissolve it in a bit of acetone, filter it through a coffee filter to get rid of any solid chunks and floaters that didn't dissolve and then mix that into the polyester... The acetone in low volume again will have nil effect on the polyester setting up, even in high volumes it will simply give the polyester a more rubber like cure vs brittle hard...
But, if you want something more 'pro' hit up Michaels, Hobby Lobby or the likes and get some Castin Craft resin pigments, they have translucent and opaque...