Hi guys,
I decided to dye a white Kastel sunshirt that I never wore… because it was white. I posted about it in a FB group and someone let me know to use RIT dye for synthetic fabrics.
https://www.ritdye.com/type/dyemore-for-synthetics/
Turned out great! Posting a pic in case anyone wants to try it. I got the RIT dye at Michael’s with a 40% off coupon and it was about $4. If you give it a whirl, make sure you follow the directions - it involves stirring constantly over almost-boiling water for 30-60 minutes. I did mine for 40 minutes.