Otherwise, if Klingons are bichromatic, then they would see any non-greyscale colour as either a variety of {Doq} or a variety of {SuD}, so it is not much possible to use colour as a defining attribute of an object. No wonder they are irritated by other species making distinctions where they see none!

I wonder if there is a Klingon word meaning "be grey". 

How a language divides up its color words has little to do with whether they can visually perceive those colors. There are real human languages that have the same number of color words as Klingons, but speakers of these languages don't lack our color vision. It's canonically speculated that Klingons can't see all of purple that we do, but otherwise their vision seems to be similar to ours.

In English we have a basic color term pink, which is considered a different color than red. But in English when we look at blue jeans, then we look at the sky, we call both of them blue. (Cyan is not a basic color term; it's comparing the color to something else.) But in Russian, for example, they use two unrelated words for the two colors. A Russian might incorrectly think that an English speaker who points to blue jeans then points to the sky and says blue for both of them cannot see the distinct colors. But we would not point at a cherry blossom and say red, even though pink is a shade of red.

When a Klingon points at a yellow plant and says SuD, then points at the green sky and says SuD, it's not that they appear to be the same color to him. It's no different than us calling jeans and the sky blue. Different shades of color grouped under one name.

In Klingon, be gray is qIj 'ej wov or qIj 'ach wov (TalkNow). Gray is a shade of black.

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SuStel
http://trimboli.name