Learn German: Das Kleid ist rosa.

In German, adjectives only get an ending when they come directly in front of a noun. Because rosa comes after the verb ist, it stays in its basic dictionary form. As a fun fact, rosa is also a special exception: it almost never takes an ending anyway, even if you put it right in front of a noun!

Does das Kleid mean 'clothes' or 'a dress'?

It specifically means a dress! English speakers often confuse it with the general word for clothing because they look similar. If you want to talk about clothes in general, you should use the noun die Kleidung.

I sometimes hear Germans use the English word pink. Is that exactly the same as rosa?

They actually describe slightly different shades in German. The word rosa refers to a light, soft, or pastel pink. When Germans use the word pink, they are specifically talking about a very bright, loud, or neon hot pink.