Those are white paper. Unless the colored insert is not to be used more than maybe a few days/weeks as the chemicals might leach out. If it is to distinguish value for selling , I would use colored round stickered outside the flip. Jim
I was hoping to find blue paper inserts, I like the way a blue background looks with silver coins. I remember at a coin show a seller had coins for sale in 2x2 flips with different colored paper inserts.
Try a hobby store, like JoAnnes Fabrics, an art suuply store, or a picture framer. They will all have a variety of acid-free artist's paper in various sizes, colors and weights. If at an art store or framer, ask for archival. quality. Then cut to size.
The colored paper inserts are the same as the white paper inserts only colored paper instead of the usual white