Q: Can I use paintstik colors on all types of fabric? Does it matter if they are cellulose fibers or protein fibers? What about synthetics?

Fiber content is not going to be an issue with paints. When you dye a fabric, a chemical bond forms between the dye molecules and the fibers. That’s why you need certain dyes for cellulose fibers (cotton, linen, rayon) and different dyes for protein fibers.
When you paint a fabric, the bond is strictly mechanical. The paint adheres to the surface of the fibers rather than forming a chemical bond. The only fabrics I know to watch out for are those that have been chemically treated to be stain resistant. If the fabric is stain-resistant, it may also be paint-resistant.
Keep in mind that almost all fabric coming into the stores is treated with finishes when it is manufactured. That is why we recommend pre-washing fabric before painting it. Use a plain detergent when pre-washing fabric and do not use fabric softener. With that single step, you should be ready to paint.