In the case of fireworks (and the powdered metals they use), you're looking at the emission spectrum of the metal in question. Combustion is a redox (reduction-oxidization) reaction, and the electron trading (ionization) involved is what causes the emission of light. read more
Titanium dioxide absorbs very little light; it only fails to act as a mirror because it has a high refractive index, so light is thoroughly scattered and mixed before being returned. (Fluorescent colors not only reflect/refract light, but also change its wavelength. read more
Hold any genuine titanium metal object to a grinding wheel (even a little grindstone on a Dremel tool will do), and it gives off a shower of brilliant white sparks unlike any softer common metal. The sparks are tiny pieces of cut titanium-the friction of the grinder heats them till they burn white-hot. read more
Titanium and magnesium sparks white. The problem with magnesium is that you can't put it out, it has to burn out. And it burns a while too. Some hardened steels spark more yellow/orange than regular mild steel which sparks bright orange. read more