In the physical world you are absolutely right MNb. 
But personally I was thinking about the mental inner feeling like some persons are associating different pieces of music with different colours. They actually see the colour when they hear the music.

A parallel in chess could be associating a colour with an opening not so much out of the name but how the pieces are moved and the formation they create. I got that feeling a little bit when it comes to KID and Dutch - not very strong I must admit.

For some weeks ago I should decide on a colour to represent KID. I choose orange nad I don´t know why.

When I am thinking of Dutch then black springs to my mind.

My favourie colours are azur blue, gold and lilac (in that order) - I don't know which chess openings though. Perhaps I may get some assistant?

"The KID is indeed orange, because that's the color of ECO-E "

That's funny...I haven't seen ECO-E.
I stopped paying attention to ECO years ago when their quality dropped.   

Its worth mentioning that Gallagher's "Starting Out: KID" and Bronstein's Kings Indian book were both also orange.  But, of course, there are many, many KID books that are NOT orange...in fact, nearly all of them. 
So I don't think this is really the source of my insanity.

But the fact remains that I see the KID as orange (and maybe someone at ECO does to. )  but I don't know why.  

Anyone else have this experience?  (i.e. openings have colors).

At the risk of sounding completely insane, I'd like to ask if anyone else
associates certain openings with certain colors.
A sort of chess 'synaesthesia'.

For example, I've always "seen" the Najdorf as Green.
I see the Kings Indian as  Orange.
The QGD is purple, the Slav is yellow...etc. etc. etc.
It helps me remember the theory of the lines, but its still a bit odd.

Its been like this for a few years, so I don't think its the onset of dementia...but who knows?

Does anyone else know what I'm talking about or should I just shut up and wait for them to come take me away?