The Charlie Brown Christmas video came out in 1965, and aluminum trees were widely available. Usually the tree was silver color, but there was a projector that made it whatever color you want. Other designs would rotate through green, red, blue, and sometimes yellow.

Here is an example from the 1962 Sears Catalog. That little lamp in the front left did the color. More types of color lamps are on page 317.

This was part of the minimalist space age esthetics of the sixties. The pendulum swung towards much richer colors in the seventies as discussed by /u/LAnatra a few weeks back.