What is Synaesthesia?
Synaesthesia is defined as “a condition in which someone experiences things through their senses in an unusual way, for example by experiencing a colour as a sound”.
There are various forms, someone with the sound colour version ‘sees’ different colours based on hearing different instruments and timbres, rather than hearing different pitches. A violin may be blue and a trumpet may be red for someone with sound colour synaesthesia, but for people with the tone colour version, like myself, it is the musical key which triggers colours. Statistically, synaesthesia of some sort is more common in people with an autistic spectrum disorder than within the general population. 
How it affects me:
The keyboard picture on this webpage illustrates the colours I associate with notes and keys. I do not physically 'see' different colours, rather I have a mental image of a particular scene, just like when music evokes memories and you are transported back to a particular moment in time and you picture that specific moment. For example, the note G is bright yellow and I associate that sound with being on a beach on a hot sunny day. I can still physically see the same colours as everyone else, so having synaesthesia does not affect my sight in any way. 
Often for me there is a mismatch between the colour of a particular piece and the memory and moment in which music takes me back to. For example the song Love Somebody by Robbie Williams on his 2002 album Escapology reminds me of a bright sunny evening, but the song for me is dark blue in colour, going against the colours associated with a bright evening. The lighter coloured notes usually correspond to happy feelings and the darker colours to sadder feelings, but this is not always true. The note F (for me which is ‘ice blue’) triggers feelings of acceptance and moving on and the note A flat (grey) evokes a feeling of longing for something.