How to make colorful marshmallow without using synthetic coloring agents?
If you are making your own marshmallows, you can add home made or purchased natural food color. With home made colors there is a trade-off: too much coloring liquid and you get the added flavor, too little, and the colors will be whitish. You make your own by concentrating the juices of blueberries, raspberries, spinach, or carrots, or by using turmeric.
If you live close to a Whole Foods supermarket (or other well supplied supermarket) look in the spice section for the India Tree natural dyes. They cost more than the artificial colors, but they save you a lot of work. You may also buy them online.