What colour eyes

I've just stumbled on this sentence What colour eyes does she have? in my grammar book. What got me interested in this is the combination of the words colour, eyes with what and without any prepositions.

I just assume a construction like this is possible with some particular words such as colour, form, shape, size and so on. These words have ranges or multiple possibilities.

Let me try this out.

What size sofa did you buy?

Since I think it's similar to what kind of car do you have construction, I don't have to use an article before sofa or do I?

What shape pancakes does the shop sell?

Are these 2 correct / possible? They make sense?

Any thoughts on this? Am I even right or it's just an insane theory? haha

UPDATE: Ok I can conclude people have different takes on this. So I want to ask you guys a couple of more things that will help me to understand this point better

1) If I always use "of" in such cases, will it always be correct?

2) What do we do with an article if we have a singular noun? optional or needed? From what I gather if we have "of" we need or opt for an article. If we don't we have to drop it. Is it like this? These below are just examples but I'm asking about a general idea for this kind of sentences
What size (a) sofa did you buy?
What shape of (a) pancake did she buy?

Solution 1:

I believe this to be a case of inversion of an accepted alternate form. In other words: many times questions are formed by inversions of statements and this is an inversion.

What color[ed] eyes did she have?

Inverts (more or less):

She had blue colored eyes.

There is a tendency in spoken language to swallow the -ed at the end of a word. And, this tends to find its way into the written form. Hence color vs. colored.

When you consider this a truncation of the -ed, you will see that a preposition is no longer necessary because it is an adjective being used as an adverb.

Consider another case:

What size shoes do you wear?
I wear size 7 shoes.

Most people don't say sized 7 shoes. So, this form stays intact in the question. And, you'd never say size 7 of shoes. So, you would be unlikely to use the form:

What size of shoes do you wear?

It just sounds awkward (at least to a speaker of AmE).

As, FumbleFingers rightly states in comments above: much of this is related to your "inner grammarian".