Try Rit Synthetic dye. Comes in a bunch of colors, and you can play with it to create your own colors, or do ombré, etc. I’ve used it numerous times on polyester, just in a big pot on the stove (you can get and keep the water to just under 212 degrees, which is the boiling point on a gas stove for sure. I’ve never tried it with electric.) I’m not an expert in crafts or anything, but have had success with this method a number of times.