A Threat Matrix uses a colour legend (high / medium / lower / no common entry). What is the purpose of colour-coding the cells?

Colour-coding shows the relative likelihood or relevance of each adversary-target-intent combination at a glance, so you can focus on the "high" cells rather than treating every threat equally.

Red typically marks high-relevance combinations (e.g. criminals targeting private companies for information theft), amber/yellow marks medium or lower/illustrative ones, and blank or grey marks combinations with no common real-world entry (e.g. terrorists targeting citizens via cyber). WHY this matters: a flat list of every conceivable threat is useless for prioritization. The heat-map view directs limited attention and budget to the adversary-target pairs that are genuinely probable for your organization.