"His jacket is pink."
Translation:Jego kurtka jest różowa.
The difference is that 'różowa' here is an adjective and the structure you're referring to applies to two nouns ( noun [nominative] jest noun [instrumental] ).
Adjectives on the other hand take the form of the noun they are applied to - here it's 'kurtka'. You could say, for example 'Jego kurtka jest różową szmatą' - 'His jacket is a pink rag' - in this example 'pink' refers to 'rag' and takes the instrumental case. I hope it's not too confusing.