Blue And Green Should Always Be Seen

I remember when I was a kid hearing this peculiar jingle, "blue and green, should never be seen-- except for in the washing machine!" I recall being stumped over exactly what this jingle was referring to. Blue and green?? Best color combo ever! Combine the two colors together and you get one of my all-time fave color combinations, along with pink and red, red and purple, turquoise and orange,