Urine was slightly red after masturbating. Any damage done?

Probably not. You were probably a little too vigorous and caused a capillary bleed in your urethra. This should clear up quickly.
Doubt, but observe.. For possible urine infection or local irritation. In females urethral irritation is easier to take place in this case as opposed to males. Also, have a level of suspicion for possibly unrelated causes e.g. Ate beet earlier or have a different reason for blood in urine like a stone disease where the color change took place as a coincidence.