"Quả chanh thì màu vàng."

Translation: The lemon is yellow.

[DmitryReve]

Everywhere else "is" was "là" but here it is "thì", why?

[Dan553966]

Under no circumstances would a native English speaker say "Lemon is yellow " with reference to the fruit. The sentence has to be pluralized or a definite or indefinite article used.