Is turquoise closer to blue or green? I am having a discussion with my coworkers. Does anyone know what is the wavelength of turquoise color and whether it is closer to green or blue when comparing their wavelengths ? 
 A: It will be difficult to arbitrate. 
Usually green color wavelength varies from $510*10^{-9}$ to $560*10^{-9}$ $m$, most commonly it is assumed $520*10^{-9}$ $m$.
Blue color wavelength varies from $450*10^{-9}$ to $495*10^ {-9}$ $m$, most commonly it is assumed $475*10^{-9}$ $m$.
Turquoise color is difficult to define, most commonly it is said to be a color between blue and green. It is similar to cyan color and in general it depends on the particular shade, but turquoise color's wavelength is usually taken as $490*10^ {-9}$ $m$, so probably it is more close to blue.
When it comes to eye's ability to distinguish colors, the just noticeable difference in wavelength for blue-green wavelengths is 1 nm. I haven't found any prove that eye's sensitivity to green wavelengths is different than to blue wavelengths.
Edit:
Under pressure of Emilio I explain - wavelength of turquoise color is not exactly equal to $490*10^{-9}$, in fact it is impossible to determine its exact value, in part because there is no clear definition of turquoise color. Here and here I have found (not particularly scientific) information about turquoise color wavelength, so I wrote that it is usually taken as $490*10^ {-9}$ $m$. 
It was supposed to be quick and concise answer and I don't think it is contrary to Emilio's answer.
A: In general it is not possible to tell, because some colours simply cannot be assigned a definite wavelength.
Some colours, like green or blue, have essentially unique combinations of wavelengths that produce the (subjective) impression of green or blue. Some colours, like yellow, can be produced in multiple ways: light of 580 nm will look yellow but so will certain combinations of red and green. Certain colours, like pink or most earth-toned colours, can only be produced by combining multiple different wavelengths, so they cannot be assigned a unique wavelength of their own.
In general, the most complete way to characterize the 'colour' of an object is not its colour but its reflectance spectrum: how much of each wavelength it reflects from a white light. For the turquoise mineral, this looks like this spectrum,

taken from

Ultraviolet-visible, near infrared and mid infrared reflectance spectroscopy of turquoise. B. Reddy, R. Frost, M. Weier and W. Martens. J. Near Infrared Spec. 14 no. 1, p. 241 (2006).

The spectrum depends on the specific sample. Reddy et al. also present the spectrum of a sample from Senegal,

which as you can see shares some features but not others. (In particular, it has a lot more content in the region from 350 nm to 550 nm, which is precisely the blues and greens, compared to the hump at the red end of the spectrum.)
