For Kant, the noumenon is not subject to the principle of individuation (which the subject contributes mainly through the senses and the intellect). That means the noumenon is single and unified, and there is no multiplicity of things within it (a multiplicity of waves, in this example). So clearly the wavelength too is not a noumenal property—whether we are talking about the "quality" of its length or an actual quantitative measurement (a given numerical value).

Of course, one can use a wave as an analogy, and then within that figurative language one can distinguish between it and its observable properties (for example, color). But that does not seem to have been the questioner's intent.

That is exactly what I wrote. There is a wave that generates the phenomena, and it has properties. A property, by virtue of being a property, belongs to the phenomenon. Still, there is room to distinguish between a property like color and wavelength. Color is obviously something that exists only in our cognition. Wavelength is not something visual, and so it can be seen as an expression of the noumenon. Any representation, visual or otherwise, of wavelength is a clear-cut phenomenon.

In my humble opinion, wavelength too is something we grasp by means of properties, such as length, size, intensity, etc.

That is precisely not what you said.
You said that "the waves and the wavelengths are the noumenon."
They are not.
Now you are saying something else (that one can see the wave as an expression of the noumenon). That is a reasonable interpretation, just not the same interpretation… And maybe one could say that your first interpretation is itself in the category of the noumenon and the second in the category of the phenomenon. Make of that what you will.

Doron,
Read to the end of the verse. At the end he writes that wavelength is indeed the noumenon, because it is not a representation of a property within us.

Yishai, you read it yourself. Read what I quoted from Michi. Hint: it's in double quotation marks in my second comment ((on the second line). Now let's see if you can be as nitpicky as I am…

By the way, I think what is especially interesting in Kant is his attempt to create a kind of middle status between phenomenon and noumenon, and in fact, in my opinion, that attempt is the very essence of his philosophy. And see how all the German idealists actually went in that direction.