Even though harvest gold appliances came out in the sixties, many people retained them through the 1970s. It's only rich and fickle types who get a new stove every time the colours go out of fashion--ordinary people make do until they need to replace them. Same for interior decor. Hair and clothes change more quickly (especially hair, which has to be cut and styled every month or two regardless), but those expensive items like wallpaper, fridges, and couches may persist over a couple of decades.
(In other words, I see nothing wrong with the art department's choices on this show, so long as no characters' exulting in how they got the latest, trendiest colour for their brand new dishwasher).