The story goes that the reason Michael Myers’ mask is flesh tone with blond hair in the school scene is because it was an unmodified Star Trek Captain Kirk mask like the one used in the original Halloween. The film producers could not afford to re-shoot the scene and replace the mask so they had to leave it in the final film.
Another version of the story says that the masked used in the school scene of Halloween 4 was THE actual converted Captain Kirk mask that had been used in the original Halloween. Over time the familiar brown hair had faded to the platinum blond seen in pt. 4. From IMDb: “The filmmakers had the intention of reusing the mask in this sequel but felt it had changed too much and decided to make their own.”
There was one other time where a Myers mask appeared in the series with blond hair. In Halloween II, Dr. Loomis and Sheriff Brackett are chasing who they believe to be Michael Myers through a neighborhood full of trick or treaters. However, you can see that the hair on this mask is actually bright blond and the skin tone is pasty white almost silver. The person behind the mask turns out to be Ben Tramer, a high school boy Laurie Strode has a crush on. As he flees Dr. Loomis and Sheriff Brackett he walks out in front of a speeding police car and is slammed up against a parked van. The van then bursts into flames, incinerating Ben.