Characterisation of the colour is often included in studies on plastic pollution. However, the comparability and relevance of this information is limited by methodology or observer subjectivity. Based on the analysis of thousands of floating plastic fragments from a global collection, here we propose a systematic semi-automatic method to analyse colours by using a reference palette of 120 Pantone colours. The most abundant colours were white and transparent/translucent (47 %), yellow and brown (26 %) and blue-like (9 %). The white colour increased in the smallest pieces (< 5 mm) and far from coastal sources (> 500 km). Both fragmentation and discolouration of ocean plastics may occur because of longer exposure time to sunlight in nature. In addition, yellow items peaked at around 1 cm and brown colours at around 1 mm, supporting the notion that yellowing precedes tanning in the aging process, which is paralleled by fragmentation. Apart from the effects of the weathering, our results suggest a second-order modulation of the colour distributions of marine plastic microplastics by the selective action of visual predators. The present work provides methodological tools and a wide empirical background to further the interpretation and applicability of the colour information on ocean plastics.