To what extent are colours intrinsically real with respect to their wavelengths?

There is NO intrinsic value in color, period.

While the wavelength of light is absolutely physically real, "color" in only a NAME that we, as humans, apply to it what we see when our eyes (and brain!) discern light in those wavelengths.

So, when confronted with light in the 400 nm range, we have agreed to call it "green".

Well, at least those of us who speak English have agreed! Those who speak French have agreed to call it "vert", and those who speak Russian have agreed to call it "zee-LYOH-niy"!!

You should be able to understand that the "what color?" is not as important as "does the physiology of the system respond to that wavelength"? Thus, an alien whos visual systems DO respond to that wavelength WILL see that "color" -- but what "color" they see will be determined by what language they speak. Thus, light in the 400 nm wavelength range way be called "b'wu'zeep" in their language. or it may be that they have no word for it at all if their eyes don't respond to those wavelengths.

Colors are not intrinsic. [See source.] What is intrinsic are the fundamental frequencies of the wave packets we call photons. And they result in real enough energy levels for the photons.

But the colors (aka, colours) are figments of our imaginations so to speak. Color is how our brains interpret the signals initiated by the photons that strike the retinas of our eyes.

And that interpretation varies from person to person; so it's not intrinsic. And color is not essential either. Some people, for example, don't see color at all. We call them color blind. Yet the photons are pretty much the same as everyone else sees.

So to your point; no, an alien species might not see the same colors as that interpretation of the frequencies would depend on how their light interpretation organs evolved and operated. Note, I avoid using "brains" as their interpretation organs might be something other than brains.

In fact, what functions as a brain might be distributed throughout the body of an alien species rather than located in one coherent organ.