<p>In altricial birds, the efficiency of parent–offspring communication relies heavily on both acoustic and visual signals, with gape colouration playing a critical role in provisioning decisions. Cavity-nesting birds breed in dark environments where the visibility and signalling function of nestling beak colouration may be constrained by ambient light conditions. In this study, we experimentally manipulated the daylight levels in the Great Tit (<i>Parus major</i>) nest boxes to test whether nestlings adjust gape colouration to varying nest luminance. Using reflectance spectrophotometry and visual models, we assessed the colour traits of four beak regions developing under natural (dark) and elevated illumination environments. In addition, we changed the light environment in the course of nestling development and investigated whether it induced beak colour modification. Our results reveal that increased nest box illumination induced physiological changes of colour in particular beak regions, such as the mandibles, which became redder when exposed to brighter conditions. Conversely, mandible redness diminished when light access was reduced. Other mouth regions exhibited age-related changes independent of illumination. These findings provide evidence of light-induced plasticity in carotenoid-based mouth colouration in avian nestlings and highlight potential constraints on visual signalling in cavity-nesting species. While gape colour varied with both age and light environment, its functional effect on parental provisioning remains to be fully investigated.</p>