The switch from platinum to beige happened in 1987. The Mac Plus became platinum and the then-new SE and II were already platinum. The first platinum Apple was the IIGS in 1986. The 512Ke remained in production but was still beige to my knowledge until it was discontinued later in 1987. White ImageWriter IIs also became platinum--their colour matched that of the Apple IIc initially.

The Apple IIe also became beige but gained a numeric keypad in the process. In addition, the IIe's closed-Apple key became an option key to match that of the IIGS (and upcoming IIc Plus). However, I believe the IIc stayed white until it was discontinued the next year.