f.3 chem

You have to use the definition of physical property vs chemical property to explain the colour change . If the color change happened without any substance changes such the rainbow which is resulted from the refraction of sunlight through the raindrops . This is the physical change . However when you look at the rusted iron , it is brownish due to the oxidation of the metal . The color change is the chemical change . In your above example , the CO2 is reacted with the lime to form a new substance which is insoluble in water . This is the chemical change .

We cannot say that whether changing colour is a physical property or chemical property because many factors are involved in it.
It may be just a physical change of temperature.
However, changing colour is usually a chemical change as the new substances affect the colour of the thing such as your example, carbon dioxide turns colourless lime water milky, it is a chemical change because calcium hydroxide react with carbon dioxide to form calcium carbonate. The formula is here : Ca(OH)2 + CO2 → CaCO3 + H2O

About your another question,
pH value is 7 is a physical property as there is no new substance is formed.

Do we consider "lead(II) oxide changes its color when subjected to different temperatures" as a physical or chemical change?

2011-10-03 11:32:02 補充：
Ha, changing of crystal structure... is that "new substance"?
diamond and graphite change their way of bonding (4 covalent bond VS 3+one pi-bond); but for ions... ?

Colour is a physical property.
carbon dioxide turns colourless lime water milky is a chemical reaction, but the physical property changed as well.