Most colors are differentiated in the segment between yellowish green and reddish orange, passing through yellow and orange.

Inside the green segment, there is little color differentiation. All the green hues between 510 nm and 540 nm wavelength look pretty similar, while in the yellow-orange segment a change in wavelength of 1 nm may cause an easily noticeable change in hue.

Also in the blue-green segment, between blue and green, there is easy color differentiation, with the hue changing strongly even for small wavelength differences.

Inside the red, green and blue segments there is little ability to differentiate the colors, unlike in the regions between these segments. This is exactly as expected, because only in the segments between the primary colors you have 2 photoreceptors in the eye that are excited simultaneously, in a ratio that is a function of the color frequency/wavelength. In the frequency/wavelength segments where only 1 of the photoreceptors is excited, the ability to differentiate hues is lost.