This question already has an answer here:
Regarding the colour of gold sol, my book says
Finest gold sol is red in colour; as the size of particles increases, it appears purple, then blue and finally golden.
This is the image I found on Wikipedia which portrays the same phenomenon.
The same page also discusses the effect of size etc. on the colour, but I don't understand exactly what's happening.
Why shouldn't the colour be the same for all sizes? Isn't it characteristic of the molecules which make the sol as the light absorbed and reflected depends on them?
Does this mean that when we break down any other thing to very small particles, it's sol will change in colour?