This episode states that there are 6 colors of the rainbow, but there are 7: Red, orange, yellow, green, blue, INDIGO, and violet.
Ms. Frizzle: This way, Arnold!
Arnold: She gets weirder all the time.
Carlos: Where did you get this book anyway? It's purple -- who ever heard of a purple book?
Dorothy Ann: It's not purple, it's violet.
Carlos: Purple, violet -- come on, lighten up.
Dorothy Ann: Fine; no problem.
(Turns off Carlos's flashlight) That lighten things up a lot. Don't ya think?
Arnold: But, Ms. Frizzle, do you really think we should be playing pinball in school? I mean, what would the principal say?
Ms. Frizzle: Mr. Ruhle. Oh, you don't need to worry about him.
Arnold: I need to worry about everything.
Arnold: Did I just see what I think I saw?
Ms. Frizzle: Arnold, you saw the light.
Dorothy Ann: Dream on, Carlos. Color is color, and light is light. Like you and me, they have nothing in common.
Carlos: Class under glass? Unreal!