What are good methods of explaining to my four-year-old son why "color" is not how he should spell "colour", even though it is spelt that way in many of the books he's reading? (Also "realize", "favorite", etc).
Similarly, although less importantly, that he should pronounce "z" as "zed" ("dance", "tomato", etc), but the characters on TV that say "zee" are right too.
He understands that there are multiple words for the same thing in other languages (e.g. counting in Māori or Spanish), but US English is so close to English that it's confusing that there are these minor differences.
I'm not saying that US English is "wrong", but at school he will be expected to spell in NZ English, and there using US spelling will be "wrong" in that it will be corrected. When he is older, he'll be able to learn why regional variants of languages exist, but for someone just learning to read/spell, that's a bit complicated.
In maths, we first learn that the square root of 4 is 2, and that is the "correct" answer in school - once we understand more we learn that it's ±2. For now, what matters is knowing that if he is asked to spell "colour" that the answer that is expected has a "u"; learning how to write in US English can be done when he's older.
I'd like suggestions as to know how I can explain what's expected, while acknowledging that the variants are (for the author) correct, without overwhelming his four-year-old brain.