Different features of pictures can be used as recognition clues. Previous studies have shown that colour is one of them. However, it is still unclear whether enhanced recognition memory by colours is due to the distinctiveness of features highlighted by colours (sensory facilitation), or it is due to the colour representation in memory (cognitive facilitation). In the last case, unnaturally coloured pictures would be more difficult to memorize. This study was conducted to investigate colour effects on picture recognition memory. Colour diagnosticity was manipulated by using pictures in two colours modes: naturally and unnaturally coloured pictures, as well as black and white pictures. Since there were three different versions of picture in the encoding phase and three in the recognition phase, there were nine possible combinations of encoding and recognizing pictures. There were three groups of participants who were exposed to three different combinations of encoding and recognizing pictures. Accuracy and recognition time were measured. Results showed strong encoding-specificity effect (better picture recognition in the same version as in the study phase), as well as improved recognition memory by both colour modes. This indicates that colour improves recognition memory through sensory facilitation, not by colour representation in memory. Moreover, recognizing pictures in black and white includes different mechanisms from those involved in recognizing coloured pictures. Furthermore, recognition time was the longest for black and white pictures and the shortest for naturally coloured ones, which suggests that recognition time depends on the fact that colour is a part of the identity of picture stimuli.