When did Yahtzee switch from red dice to white dice?

The history of "Yahtzee" suggests that it took its style of play from other dice games such as "Generala", played in Puerto Rico, and 'Poker Dice' and 'Cheerio' from England. But the most similar game to it is a British game called "Yacht".
Milton Bradley made a few changes before he came out with the game "Yahtzee" in 1956, which was in turn marketed by E.S.Lowe from 1956 until 1973 when it was purchased by the toy company, "Hasbro".

Originally it came with red dice and it seems they changed to white after the games' rights were purchased by Hasbro. Although there's no direct evidence to prove that, the red dice definitely disappeared from the games with the first boxed set that came from Hasbro's factory.