Re: What makes chemicals change the color of fire?

We see chemicals, and other things, to be colored when they emit or 
reflect light in the region of the electromagnetic spectrum called visible 
light. Chemicals emit light when an electron falls from an orbital to 
another orbital closer to the nucleus. 

The heat of a flame causes some of the electrons, especially those in "d" 
and further out orbitals (principal quantum number n = 3 is the first 
orbital to have a "d" subshell) to be raised to a higher level, or to be 
completely removed from the atom. When the atom moves to a cooler part of 
the flame, it is now energetically favorable for an electron to drop back 
into that orbital. If the energetics are such that the energy given off is 
in the visible light part of the spectrum, we see color. 

If salt is heated, and sodium ions are formed, when an electron falls back 
into the 3s orbital, the yellow light we are familiar with is emitted. 
Generally, it is the transition metals that emit colors because they have 
orbital transitions that are in the visible light range.

If you would like more information, your library might have any first year 
general college chemistry text which always has a section on electronic 
structure, and discusses the colors of the elements.