Is it allowed to drop that ''색'' when we use the color as an Adjective / Noun Modifier???
Example A/ The cat is black: 고야이가 검정색이에요.
Example B/ Black cat: 검은 고야이.

As we know, the black color has 2 forms: 검정색 and 검은 (Adjective the know Noun Modifier) .

What about other colors that have one form such as 보라색? is it possible to drop the 색 and use just 보라 as an adjective?

Which one is correct in that case??
보라색 컵이에요
보라 컵이에요

Question

Is it allowed to drop that ''색'' when we use the color as an Adjective / Noun Modifier???
Example A/ The cat is black: 고야이가 검정색이에요.
Example B/ Black cat: 검은 고야이.

As we know,  the black color has 2 forms: 검정색 and 검은 (Adjective the know Noun Modifier) .

What about other colors that have one form such as 보라색? is it possible to drop the 색 and use just 보라 as an adjective?
 
Which one is correct in that case??
보라색 컵이에요
보라 컵이에요

Answer

검정색 is grammatically wrong, and even many native speakers aren't aware of this.
It should be 검은색

If you see 'ㅇ' as the supporting consonant at the end (노랑, 파랑) you shouldn't attach '색' since they are already nouns.

As for these "primary colors"(common ones), both are correct, but the nuance is slightly different.

빨간 옷 : generic red cloth (any red)
빨간색 옷 : cloth in a particular red tone. (further detalis may follow)

And 보라 is already a very particular color. (compared to the more common ones like red)
Hence it shall be "보라색 컵"

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