Colours are physical properties of external objects. One such colour is Pure Green: the shade of green that looks not at all yellowish or blueish. However, if people are asked to identify the shade of green that looks not at all yellowish or blueish, they come up with (slighly) different shades: what looks pure green to me looks slighly blueish to you; what looks pure green to you looks slightly yellowish to me. What shall we make of this?
We could claim that one of the groups is simply right about Pure Green and the other wrong, even though there is no way to find out which is which. That is incredible.
Alternatively, we could say that colours are relational properties: something is pure green only relative to an observer. That's not entirely incredible, but also suspicious. It seems to confuse the way we pick out a colour with what we thereby pick out.
Compare two thermometers A and B of which whenever A displays a number n, B displays n + 23. It would be silly to conclude that temperatures are relations to thermometers.
Is that because, in this case, (at least) one of the thermometers simply gets the temperature wrong? Not necessarily. We could also say that they measure temperature in different units: A in degrees Celsius and B in degrees Celsius + 23.
I think we should treat the case of Pure Green in the same way: what I call "Pure Green" is a physical property, not a relation to observers. It is the very same property you call "slightly blueish green". That we give them different names doesn't prove that they are different properties. It merely indicates that we pick out the property differently.
In my childhood, we had a hoover with a distinctive greenish colour we called "hoover green" ("staubsaugergrÃ¼n"). Other people perhaps used that same term for a different shade of green. This doesn't show that Hoover Green is not a real colour -- that it is a relation between various shades of green and people or hoovers.
Suppose hoovers were available in all shades of green, but never in red or blue or yellow, and "hoover green" became a common term that everyone used for the colour of their (or their household's) hoover. People would apply "hoover green" to different colours, but it would be ordinary colours they applied it to. Of course, "hoover green" wouldn't be a terribly useful term. If I told you that I painted my wall hoover green, you wouldn't know what shade of green I painted it unless you knew the colour of my hoover.
"pure green", I think, works like "hoover green", except that people wrongly assume that everybody's hoover has (almost) the same colour. We all use "pure green" for the shade of green our visual system processes in such a way that it looks not at all yellowish or blueish to us, and most of us wrongly believe that this is the same shade for us all. Well, in fact, I suppose "pure green" is hardly used at all; I should really speak of "the shade of green that looks not at all yellowish or blueish". This is also hardly used, but we have determinate intentions and expectations towards its use; for instance, members of our community are disposed to believe that a certain wall has a shade of green that looks (to them, and to all ordinary people) not at all yellowish or blueish if a trustworthy person tells them, "the wall has a shade of green not at all yellowish or blueish". The error is in the brackets: most people, I suspect, assume that what looks pure green to them looks pure green to most others as well.
Is this error built into the definition of "pure green"? That is, once we notice that people classify slightly different colours as pure green, should we conclude that there is no such colour as Pure Green at all? Or should we conclude that "pure green" is an indexical colour term like "hoover green"? Well, since "pure green" is kinda technical, we're free to choose, but I think the second choice is more natural. "pure green" thereby becomes less useful than we might have hoped: if I tell you that I painted my wall pure green, you don't know what precise shade of green I painted it unless you know what colour looks pure green to me.
(I should add that I think "pure green" is a very special case. Most other colour terms, like "Brunswick green" and "#00a913" and "green", are not defined by how they look to us, hence there's no danger that they might turn out to be uninformative indexicals.)
(I should also add that I am not at all familiar with the literature on colour. I've only read "Color Realism and Color Science" (PDF) by Byrne and Hilbert and the replies to it (also in the PDF); B&H argue for the incredible thesis that most people are wrong about Pure Green, while some of the replies argue that one should instead construe colours as relations. The solution I favour doesn't come up, but I'd be surprised if it is new. Maybe it faces serious objections I fail to see?)