This is true. But even within this window, e.g. between 1100 nm infrared and 700 nm red, we could put 40 different "colors" at 10 nm steps. Separation at the receiving end may become hard though.
Standard ITU grid is 100 GHz channel spacing, with subdivisions of 25 and 50. We're routinely using symbol rates high enough that the channels are fairly well filled.