What color was the unblemished heifer whose ashes were used for the water of purification?

Answer: Red

Israel was to bring an unblemished red heifer that had never been yoked; its ashes were kept for the water of purification.

““This is the statute of the law that the LORD has commanded: Instruct the Israelites to bring you an unblemished red heifer that has no defect and has never been placed under a yoke.”— Numbers 19:2 (BSB)