Learn Russian: Это платье розовое.

You would use the question word какое. Because платье (dress) is a neuter noun, you must use the neuter form of the interrogative modifier какой. The question would be Какое это платье?

Why does the word for pink (розовое) end in -ое?

Adjectives must match the gender of the noun they describe. The noun платье is neuter because it ends in -е. Therefore, the adjective takes the neuter ending -ое. If the noun were feminine, it would be розовая, and if masculine, розовый.

Could this sentence also mean 'This is a pink dress'?

Yes, it could! Because Russian doesn't use words for 'is' or 'a' in the present tense, Это платье розовое can mean either 'This dress is pink' or 'This is a pink dress.' However, to say 'This is a pink dress,' it is much more common to put the adjective before the noun: Это розовое платье.

How do I pronounce all the 'о' vowels in розовое?

The stress in розовое is on the very first syllable. In Russian, an unstressed о sounds like a short 'a' or 'uh'. This means only the first о sounds like a true 'o', while the second and third ones are reduced.