Our ability to perceive changes in wavelength depends on several variables, most important of which are the wavelength itself and the luminance level. As a general rule of thumb, it is reasonable to say that we are able to detect changes in wavelength of about 2nm in the middle of the visible spectrum. At the ends of the spectrum this number increases and it becomes undefined at the very ends of the visible spectrum where only a single type of cone photoreceptor is responding to the stimulus. Some classic measurements of wavelength discrimination are given in Wyszecki and Stiles, Color Science, 2nd Ed. on pages 570 and 571. The show, among others, results from a 1934 study by Wright and Pitt indicating that wavelength discrimination was as good as about 1nm near 500 and 600nm, around 2nm for much of the visible spectrum and then in excess of 5nm for wavelengths below 430nm and above 650nm.