for tv shows, a 1 khz wave 3x with a small O lower right. beep, beep boop, every 1 second then 4th second is record. This builds a working tempo both visually and aurally so they don’t jump in too soon or too late. often we don’t even use a visual symbol. most actors are good with the beeps. BEEP!
changing color gives a subconscious drag race effect and creates stuttering. imho.
a small tip, don’t blind the actor with a bright picture. they will be staring at it a lot.