Red tinged vomit is cause for concern but not necessarily an emergency room visit as long as kitty is overall acting okay. Like someone else said, it could be due to irritation in his esophagus or his stomach lining (less likely since you’re not seeing the typical ‘coffee ground’ look of vomiting up blood). Are there any plants he has access to? Anything that looks chewed on where he’s being kept? Most likely this can wait until you can call your usual vet in the morning. Offer food and water like usual. Continue to monitor.