Harry Beck's 1931 London Underground map established the core rule: each line gets one color, that color means only 'this is that line', and no color has a secondary meaning in the system. This single-meaning rule is violated constantly in digital design, where the same blue is simultaneously a link color, an information state, a brand color, and a navigation highlight.
Healthcare wayfinding research finds the most effective systems use 6-8 zone colors maximum, pair every color with non-color redundancy (floor numbers, symbols, text), and never rely solely on color for navigation. Approximately 8% of males have some color vision deficiency — wayfinding color is always a confirmation, never the sole cue.
In digital multi-product suites (Google, Microsoft, Atlassian), each product gets a single color that appears in the nav chrome, favicon, and document headers — meaning only 'you are in this product' and nothing else. This wayfinding-correct approach explains why these color systems feel coherent despite spanning hundreds of products.