Metang's hand glows white, and it punches the opponent. When the punch makes contact with the opponent, orange, yellow, and pink meteor-shaped particles come out of the white light on Metang's hand.
Metagross's hand glows white or a sillouette of a golden meteor appears around its hand and it punches the opponent. When the attack makes contact, multicolored or golden meteor-shaped or star particles come out of the white light on Metagross's hand.
The ability Iron Fist (which boosts the power of punching moves by 20%) affects Meteor Mash. This is because the Japanese name of Meteor Mash is Comet Punch (which was already taken by a Generation I move).