Over sixty different types of Synaesthesia have been discovered. Colour synaesthesia is known as Grapheme, or Colour-Graphemic Synaesthesia. Another common form is Ordinal Linguistic Personification. An example of this would be a month of the year, having its own personality (I can imagine it being extremely interesting!). Synaesthesia is genetic, and I believe it is more common in families with twins or triplets. Some scientists believe that it is so common that up to one person in every twenty three people has one type of synaesthesia.
Synaesthesia with colours can be extremely helpful if you need to remember a name, because quite often that person will have a colour, and you will remember their colour (their colour will usually be related to the first letter, or last letter of their name) i.e. for me Catherine is yellow because my C is yellow, and my E is yellow. So if I ever forget Catherine’s name, I will know her as a yellow person, and so I will go through my yellow letters e.g. E, C and S, and that might help me to remember.
A lot of famous musicians (such as Adam Jones, who was Tool’s guitarist) have synaesthesia too. That type of synaesthesia will usually be the association of notes or pitches of sound to a colour. It is recorded as being something similar to fireworks in your mind when you hear music. I can imagine that being very helpful when it comes to writing music. Synaesthesia is a gift, and not a disorder, although lots of people hide it if they have it, because they believe that they are just being crazy.
Older reports say that synaesthesia is much more common in women than in men, the ratio being almost 6:1 with every six women having it, only one man will. This is probably due to a sampling bias, because more women are likely to admit to it than men. More modern reports tell that the ratio is equal.
It is possible for synaesthesia to skip generations, and also skip people. It has been found that in some cases of identical twins, one twin will have synaesthesia, and the other twin will not! Synaesthetes (the name given to people who have synaesthesia) are far more likely to be ambidextrous or left handed, and baby boys, who are born with synaesthesia, have a slightly bigger chance of being a miscarriage, a worrying statistic. Many scientists have started to conduct research on synaesthesia (after research stopped in the early 20th century) and it is becoming a much more understood topic.