The title of this page is not exactly right, because topics will include emission of light from atoms and molecules, with various causes.

If you throw certain things on a bonfire or put them in fireworks (first picture), you may see brilliant colours.  Sodium makes yellow, as in street lamps.  Copper makes greenish blue.  Strontium gives a reddish colour, neon orange, and so on.  These are the colours that are emitted by atoms of the substance when they are given energy.  By means of these signatures, astronomers can infer the presence of substances in stars that are at unimaginable distances.  

The second picture above shows a mercury lamp and its spectrum.  Quite clearly it is is not continuous; there are bright lines in the blue, green, yellow and red parts of the spectrum.  So although the resulting colour is quite close to white, it does not mimic daylight very well.  It would not be useful for colour matching of cloth.

The yellow light from sodium is particularly pure - the bandwidth is very narrow.  So most things look very odd in sodium light.  If the yellow were composed of a mixture of red, orange, yellow and green, a far wider range of objects could reflect at least some of the light.  

There are many ways of making atoms and molecules emit light.  If your computer has a CRT, the light is caused by fast electrons hitting phosphors.  A TFT screen also emits light as a result of electrical energy.  In many substances the colour of the light is due to the structure of the crystal as well as to the types of atoms.  Some impurities can change the colour of a substance dramatically when present in very small amounts.  This happens in crystals and in glasses.