Light in WDM system falls under near infra-red region and it is invisible.
To differentiate wavelength in multiple systems, colored light term is used in WDM system wheres grey light is used in common optical systems.
Colored light is WDM-side optical signal either in OTN or line boards in WDM system. The optical signals can be directly transmitted to the multiplexer since they have already standard and dedicated wavelength.
Grey light on the other hand has no standard wavelength and it is within a certain wavelength range. Normally, light at Client-side optical ports of WDM device is grey light.
