A quite rare occurrence, the exact explanation still remains elusive. Some theories suggest that this may happen due to mixing of neural connections in our brains. A neuron, which transmits information to our brain may see the color red and send the information to our brain. Due to mixing of information, another neuron which knows the smell of mango can get associated with this color. So our brain gets both the information at the same time: the color red and the smell of mango. People with this condition therefore can smell the color red.