[steelrat1 (OP)]

When someone says think of a number,
what colour is it?
What colour do you see it in your minds eye?
How do we assign  colour to shapes or numbers?
Do you see it in colour or just a clear shape.?

[chiralSPO]

Some people do have very strong associations between colors and numbers to the extent that it can be called synesthesia--an actual perception of one sense (visiual, aural, tactile, etc.) in response to a different sensory stimulus or thought. Unfortunately synesthetes don't agree on which color a given number is.

[steelrat1 (OP)]

“How do we assign  colour to shapes or numbers”   what I mean by this question is  how does the brain make a complex decision on what colour is associated to what number.

[evan_au]

The cause for synesthesia is not proven - some researchers think that it is cross-wiring of the sensory circuits in the brain. It certainly has a genetic component to it.
See: https://en.wikipedia.org/wiki/Synesthesia#Mechanism

If you pick a particular mode of synesthesia (eg number↔colour), less than 1% of people have it.
- But the number of potential sensory combinations is large, and so the percentage of the population with some form of synesthesia could be over 5%
- One study focusing on 9 potential modes found an incidence of 4.4%
See: https://en.wikipedia.org/wiki/Synesthesia#Prevalence

[Petrochemicals]

The cause for synesthesia is not proven - some researchers think that it is cross-wiring of the sensory circuits in the brain. It certainly has a genetic component to it.
See: https://en.wikipedia.org/wiki/Synesthesia#Mechanism

If you pick a particular mode of synesthesia (eg number↔colour), less than 1% of people have it.
- But the number of potential sensory combinations is large, and so the percentage of the population with some form of synesthesia could be over 5%
- One study focusing on 9 potential modes found an incidence of 4.4%
See: https://en.wikipedia.org/wiki/Synesthesia#Prevalence

Given that the number 8 in Asia is lucky and associated with green for money in gambling. This is cultural linking of specific colours for specific situations.

https://www.chinahighlights.com/travelguide/culture/lucky-numbers-and-colors-in-chinese-culture.htm.

[chris]

This is quite a helpful summary on what is synaesthesia?