We know that "The bag is black." is a correct sentence. But, a lot of people write "the bag is black colour". Is this sentence grammatically wrong or acceptable?
Your first (valid) example of "The bag is black" uses 'black' as an adjective.
It would also be valid to say "The bag is black coloured"; here the word 'black' is used as a noun modifier for the adjective 'coloured'.
Also idiomatic would be "The bag is black in colour" ('black' as adjective, 'colour' as noun).
"The bag is [or has] a black colour" is also fine, as 'black colour' acts as a compound noun being used to express a quality, property or relationship of the bag (similar to "The tube is a cylinder", "The car has a door", "The man has a child").
But in "The bag is black colour" you're saying the noun and the compound noun are equal, which they are not.
*The bag is black colour
This sentence is ungrammatical. The reason is that colour is a singular countable noun. Singular, countable nouns in English must have a determiner:
- *I have pen
- I have a pen/ the pen /my pen/ this pen/ one pen/ John 's pen / which pen/ any pen
So you can say:
- The bag is a black colour
or you could use the adjective coloured
- It's black coloured.
However, neither of these is very good style, because everybody knows that black is a colour!