Roughly the range of wavelengths from 620 nm to 780 nm is seen as red to human eyes. So the color of 620 nm and 780 nm can be broadly categorized as red but due to a large difference in the wavelengths both will not look same.
The range of wavelengths roughly from 590 nm to 620 nm is seen as orange to human eyes. But the color of 620 nm and 621 nm light may not be differentiated by the human eyes. Because the range is roughly divided, the 620 nm light and 621 nm light will appear same.
In the white light of Sun broadly seven colors are there. Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR), but a large number of shades of each of the colors can be distinguished by human eyes.
