Learn Polish: Herbata jest dobra, ale zimna.

In Polish, ale is used to show a direct contrast or a surprise, just like "but" does here: the tea is good, but disappointingly cold. The conjunction a is used more for comparing two different things, like saying "while" or "and" (for example: Kawa jest gorąca, a herbata jest zimna - Coffee is hot, while tea is cold).

Why do both dobra and zimna end in -a?

Adjectives in Polish change their endings to match the gender of the noun they describe. Because herbata (tea) is a feminine noun, both adjectives describing it must take the feminine dictionary ending -a.

I remember learning to change the ending after jest (using the Instrumental case). Why is it dobra and not dobrą?

You use that changed ending (the Instrumental case) after jest when you are linking a noun to another noun (like "She is a teacher"). But when you only have an adjective after jest, as in this sentence, the adjective stays in its basic dictionary form (the Nominative case).

Is it grammatically correct to say ...ale jest zimna and repeat the verb?

Yes, you absolutely can! Herbata jest dobra, ale jest zimna is completely correct. However, just like in English, Polish speakers often drop the second jest to make the sentence shorter and more natural.