Colour in a design interface does real communicative work on its own, entirely separate from whatever text or imagery it happens to accompany. A red button typically reads as carrying warning or urgency, and a muted grey one typically reads as inactive or disabled, and this reading generally registers with a viewer before they've consciously processed the label sitting on top of it. This isn't decoration; colour functions as one of a design's fastest available channels for communicating meaning and hierarchy.
Colour associations let a viewer process meaning faster than reading text
Within a given cultural context, certain colours have become fairly strongly associated with specific meanings through widespread, consistent use, red with danger, warning or stopping, green with permission, safety or success. Because these associations are already well established in a viewer's mind before they even encounter a specific interface, a designer can use colour to communicate a state or action's basic character essentially instantly, well before a viewer has had time to read and process any accompanying text label, which makes colour an unusually fast communicative channel compared with text alone.
Colour also establishes hierarchy, showing which elements matter most
Beyond conveying specific meanings, colour also establishes visual hierarchy within a design, guiding a viewer toward which elements are most important and deserve attention first. A bright, saturated colour applied to a single primary action button, surrounded by more muted, lower-contrast colours elsewhere on the same screen, draws a viewer's eye toward that primary action specifically, letting colour do real structural work in organising an interface's priorities, not merely making individual elements look attractive in isolation.
What we're still unsure about
That colour carries meaning largely independent of accompanying text, and that it plays a significant, well-documented role in establishing visual hierarchy, are well established, extensively applied principles across design practice and research. What's more genuinely contested is exactly how universal specific colour-meaning associations actually are across different cultures, since colours that read as one thing in one cultural context can carry a genuinely different, sometimes opposite, association elsewhere — designers working across cultural contexts continue to navigate this variation carefully, and design researchers continue studying how much of colour association is genuinely universal versus culturally learned, rather than treating any single colour's meaning as fixed and globally consistent.
This sits inside Colour in Design, one of seven topics in Design, one of five domains in Art, one of seventeen subjects the app can quiz you on.