Here is a simple grammar check and a clearer version you can use.
Original sentences:
A flamingo's feathers turn pink if it eats red shrimp. Without the shrimp, flamingoes are light grey.
Corrected (simple) version:
A flamingo's feathers turn pink when it eats red shrimp. Without shrimp, flamingos are light gray.
Short notes for a 6-year-old:
- "When" instead of "if" says this really happens whenever the flamingo eats the shrimp. (Using "if" was not wrong, but "when" is a little clearer.)
- "Flamingos" is the more common plural spelling ("flamingoes" is also okay).
- "Gray" and "grey" are both correct — "gray" is American spelling, "grey" is British spelling.
So you can pick either spelling for the color, but the sentence above is a clear, simple way to say it.