The Radio Room Clock with its coloured segments were one of the specialised clocks. The coloured segments, each measuring 3 minutes long indicates periods which radio silence observed. This is to facilitate listening to any distress calls on the International Distress Frequencies (for many years 500 kHz was the primary international distress frequency).
The red segments indicate the listening period for distress calls made with Morse code, while the green segments indicate the listening periods for voice calls on (usually) 2182 kHz (medium range maritime voice use). Other frequencies include 121.5 MHz for civil aviation and 243 MHz for military use. Current international frequency has been updated to 406 MHz (digital) — via Horology Crazy