Previous literature has explored unconscious racial biases in clinical education and medicine, finding that people with darker skin tones can be underrepresented in learning resources and managed differently in a clinical setting. This study aimed to examine whether patient skin colour can affect the diagnostic ability and confidence of medical students, and their cognitive reasoning processes. We presented students with 12 different clinical presentations on both white skin (WS) and non-white skin (NWS). A think aloud (TA) study was conducted to explore students' cognitive reasoning processes (n = 8). An online quiz was also conducted where students submitted a diagnosis and confidence level for each clinical presentation (n = 185). In the TA interviews, students used similar levels of information gathering and analytical reasoning for each skin type but appeared to display increased uncertainty and reduced non-analytical reasoning methods for the NWS images compared to the WS images. In the online quiz, students were significantly more likely to accurately diagnose five of the 12 clinical presentations (shingles, cellulitis, Lyme disease, eczema and meningococcal disease) on WS compared to NWS (p < 0.01). With regards to students' confidence, they were significantly more confident diagnosing eight of the 12 clinical presentations (shingles, cellulitis, Lyme disease, eczema, meningococcal disease, urticaria, chickenpox and Kawasaki disease) on WS when compared to NWS (p < 0.01). These findings highlight the need to improve teaching resources to include a greater diversity of skin colours exhibiting clinical signs, to improve students' knowledge and confidence, and ultimately, to avoid patients being misdiagnosed due to the colour of their skin.