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The visible red light spectrum stretches from 620 nm to 780 nm. So the color that a human eye will perceive is same for 620nm and 780nm but it will be a different shade of red.

Human eye cannot distinguish between minute wavelength differences. Hence 620nm and 621nm will appear almost same to a human eye. Though for a high performance spectrum detector, both these wavelengths will be perceived as different colors.

The white light is roughly divided into a spectrum of seven colors. These are Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR). There is an entire spectrum of colors present in between two such colors, some even easily distinguished by a human eye.