Why is «Какого цвета» in genitive case?

Question

I came across the sentence «Какого цвета платье?» today. I would've assumed that «Какого цвета» would be in the nominative case and was confused to see this construction. Is there a grammatical rule/logic behind this, or is it just another case of "they just say it this way, so learn it".

Answer 1

Nominative doesn't work for the same reason it doesn't work in this English statement:

• *The dress is red colour.

But we can make it work using 'of':

• The dress is of red colour.

I don't know why English loses the 'of' when the statement is converted into a question:

• What colour is this dress?

Other languages keep it:

• De quelle couleur est cette robe?
• Di che colore è questo vestito?

So it's English that's the odd one out. Russian is logical. :)

Answer 2

In Russian, the question about the color of the dress has at least 3 possible answers:

(1) Платье красное. ~ Красное платье.  
(2) Платье красного цвета. ~ Красного цвета платье.
(3) Цвет платья – красный. ~ Красный цвет платья.

Now let us see how we construct a question to get an answer of type (1), and how we do it for types (2) and (3).

In (1) красное is a Nominative case singular neuter gender qualitative adjective, so in our question we will need the question word какой? in the same form (Nom. sg. n.) — какое?

– Какое платье?
– Красное платье.

However, какой? can stand for any kind of a qualitative adjective, not only for a color, but also for size, length, age, price, etc., which means that instead of an answer about the color of the dress we can well get answers about its other qualities, e. g. Длинное платье. Новое платье. Красивое платье. Дорогое платье.

To make sure the answer will be exactly about the color of the dress, we must use the word цвет “color” in the question, which means type (2) answer will be expected. In (2), красный цвет “red color” is an attribute of платье, that is why the noun phrase красного цвета is in the Genitive case, in the case of an attribute. To make a question, красного is substituted for какой?, but this time in Gen. sg. n. — какого? (of what color [is] dress?):

– Какого цвета платье?
– Красного цвета платье.

Красное платье ~ Платье красное (adjective in the Nom. case) can also be an answer to that question.

When we need an answer of type (3), a questions about color can be asked with какой цвет?, Nominative. This time цвет is the head of the noun phrase and платье is its attribute in Genitive (what [is] color of dress?):

– Какой цвет платья?
– Красный цвет платья.

Irrespective of which of the types you choose, you cannot have both какой цвет? and платье in the Nominative case, one of them has got to be put in Genitive.

Answer 3

Sergey Slepov's answer is correct (though I'm not agree with languages weird-shaming).

Note also that Russian doesn't use the verb have too much.

So this construction is less usual for Russian:

The dress has this color. ACC

While this one is more usual:

The dress is of this color. GEN

And that's why the question is also in GEN.