It is possible to have a 10 color rainbow, with the additional 3 colors made up of combinations on the ROYGBIV spectrum. The colors we see in a rainbow are just our perception of the wavelengths that bounce off of it. Some people can see additional colors on the ultraviolet spectrum, in a condition known as aphakia. However, this condition arises due to surgical removal of the lens of the eye, and is fairly rare. Otherwise, humans vision is typically confined to the visible spectrum. Hope this helps!