Wikipedia defines colour as follows:
...is the characteristic of human visual perception described through color categories, with names such as red, orange, yellow, green, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum.
Colour is then defined solely in terms of human visual perception, so if 620 nm or 621 nm correspond to different colours or hues of colours depend on our capacity to tell any difference between those wavelenghts. Also, the visual spectrum of light is said to be divided in seven colours because we can distinguish seven big categories in the visual spectrum, and that can be analyzed as a result of the phenomenon of scattering of white light through a diffraction plate, and it looks as follows (I couldn't add the full red range, but it doesn't really matter):
Realistically, All wavelengths should differ in shades considering the red hue increases in every colour as wavelength increases. Then how come we say that only Seven colours form White light?
The important thing here is that this spectrum is a $\textbf{continuum}$, which results in no clearly defined limit or frontier between colours because of the smooth color gradient that exists. Looking at this spectrum the first thing that we can tell is that there are seven big categories that result in a range (continuum) of wavelenghts that we can very easily distinguish: purple, blue, cyan, green, yellow, orange and red. Looking for example at 550 nm and 600 nm in the previous figure, we can say that both wavelenghts correspond to the same "bag" of wavelenghts which we name $\textbf{green}$, but if we carefully observe
those colours we note that we can actually tell that they are $\textit{different}$ hues of green because of the shading.
Do all wavelengths between A to B correspond to Yellow light? So, (A+1) nm, (B-1) nm, along with A nm & B nm are all exactly of the same colour ? Is there any difference even in their shades?
If A and B define the yellow range, then all colours between A and B are said to be a hue of yellow. Take the wavelenghts that approximately corresponds to A nm, and go 1 nm away from it. Both wavelenghts are certainly inside the "yellow bag" due to the range we define. As this is a continuum spectrum that gradually changes, one can argue that A nm and A+1 nm are in fact $\textit{different}$ in the physical sense that they actually correspond to different wavelenghts, and that is in fact the property that characterizes electromagnetic radiation (besides intensity, which is related to the sensitivity of the eye and it is not of much importance in this discussion), but if I handle you a pair of objects that reflect $\textit{only}$ A nm and A+1 nm wavelengths, then it is almost certain you that you will not be able to tell the difference between them. This way, in practical terms relative to human perception, both colours are the same kind or hue of yellow. If you go 1 nm away to the left of A nm so you leave the yellow range, a person could not be able to tell the difference at the same time that other may say that it actually corresponds to the green range, and this is one of the subjectivities that arise in defining the limits in these ranges of colour. This way you see that if a pair of wavelenghts is distinguishable (different) for human eye is not unequivocally defined.
Does '(B+1)' nm of wavelength straightaway correspond to Orange light ? Or Is the orange colour going to build up as wavelength increases from B ?
This is the same case as the limit between green and yellow range previously discussed. When you $\textit{define}$ the yellow range to correspond to wavelenghts between A and B nm, something like $\lambda_{yellow} \in [A,B]$ (it doesn't really matter if it is a closed or open interval), then if you accept to consider seven different categories in the visual spectrum (the colours usually used) you can say that wavelenghts inmediately after B nm are in the orange range, even though B-0.001 nm and B+0.001 nm are indistinguishable by the human eye.
Whenever you talk about colours in the usual sense (not making precise light measurements or anything like that), you usually don't care about 1 nm difference, and that is one of the advantages in defining these categories qualitatively, so there are no problems like those that arise in the frontiers. In the physics realm however, one might talk about wavelenghts or frequencies instead of colours, so again we don't have to worry about ambiguities.
