Current-generation ROADMs are limited by fixed add/drop transceivers and wavelength assignments. When a wavelength is assigned, a transceiver must be physically connected to the correct mux-demux port at the add/drop site. Even if the transceiver is tunable, it still must be manually moved.
A colorless ROADM allows this process to be automated. But to achieve this, several wavelength-selective switches (WSSs) are needed at the add port. The number of WSSs used is set by the port count: For an 80-wavelength add, up to nine 1x9 WSSs are used. Higher-degree 1x23 WSSs will reduce the units needed while also potentially lowering the cost-per-port.
On the receiver (or 'drop') side, vendors use multiple WSS ports or power splitters. But with the emergence of tunable coherent receivers at 40 Gbit/s and 100 Gbit/s, colorless demultiplexing using a coherent receiver to select the desired channel is possible, simplifying the ROADM design.
Regardless of the architecture, the end result of a colorless design is that any wavelength (color) can be assigned to any port of the add/drop site using software, and without a technician having to be on site.