Lightness Before Hue: What the Evidence Says About Colour in Digital Learning Interfaces, with Design Guidelines and a Study Plan for Indian Secondary Students

Designers of study apps often assume that bright, colourful interfaces make students happier and that dark interfaces make them gloomier or protect their eyes. We review the evidence behind these assumptions for the design of a study app used by Indian students in Classes 9 to 12. The review is a structured narrative evidence review across five areas: colour and emotion, colour and learning (signalling, colour coding and emotional design), screen polarity and visual fatigue, accessibility, and the Indian context. Each claim is graded strong, moderate, weak or not supported. Three findings are robust. First, colour attributes matter more than hues: saturation raises arousal and captures attention, and lightness raises pleasantness. Popular hue effects, such as red impairing test scores or blue aiding creativity, failed large replications, and the red effect falls to about zero after correcting for publication bias. Second, colour helps learning when it codes meaning. Signalling and colour-coding meta-analyses report gains of roughly d = 0.3 for transfer and 0.3 to 0.5 for retention, concentrated in novices, while decorative additions cost about as much (g = −0.33). "Emotional design" with warm colours and round shapes gives learning gains near 0.3, but the mood change that is supposed to explain them is near zero, and colourfulness alone did not improve learning in a recent upper-secondary experiment. Third, dark text on a light background gives more accurate proofreading in adult lab studies, an effect of display luminance that grows as text gets smaller; there is no good evidence that dark mode reduces eye strain or that dark interfaces lower mood, and night-tint features did not improve sleep in a randomised trial. For Indian students the strongest case is practical: red–green colour vision deficiency affects roughly 2 to 4% of schoolboys, urban child myopia has risen to about 21%, and most phones are budget devices used in daylight. We translate the evidence into graded design guidelines and a six-study research plan, and we note that no study we found tested colour or polarity on learning with Indian secondary students. This paper reports no new data.