= Glass colouring
The Sun's rays are not monochromatic and represent photons of various wavelength that may or may not be part of the visible light. In glass, some ions absorb particular wavelengths in the visible light and emit only the remaining wavelength. This is what contitutes the colour of an object. If an ion absorbs photons of red light, it appears bluish. These ions are normally transition metals as they can absorb photons of wavelengths that we can see.
The colour that we perceive by the glass is not only related to the ions contained in the glass but it also depends on the network-former/modifiers present. This is because the ions interact with them, which influences each ion's orbital energy levels and consequently the light that they can absorb. Lastly the colour also depends on the concentration of the ion, its nature (valency) and the heterogeneity of the glass. A portion of the light that arrives in the surface of the glass gets reflected and doesn't interact further.
A list summarizing some generally perceived colours and their associated ions:
* $Fe^{3+}$ ($Fe_2O_3$) $\rightarrow$ yellow-brown
* $Co^{2+}$ ($CoO$) $\rightarrow$ blue
* $Cu^{2+}$ ($CuO$) $\rightarrow$ blue
* $Cu^{+}$ ($Cu_2O$) $\rightarrow$ red
* $Cr^{3+}$ ($Cr_2O_3$) $\rightarrow$ green
* $Cr^{6+}$ ($CrO_3$) $\rightarrow$ yellow
* $Ni^{2+}$ ($NiO$) $\rightarrow$ green
* $Mn^{2+}$ ($MnO$) $\rightarrow$ yellow
* $Mn^{3+}$ ($Mn_2O_3$) $\rightarrow$ violet
Glasses can also be coloured by metal colloids or to coloured particles.
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