Colored Salt Flame Tests
Metal salts can be used to color flames a range of brilliant colors, from green to purple. It's a fun experiement that can be easily done at home.
WARNING: Most metal salts (except for compounds like NaCl) are toxic. Don't eat them. And putting some of the chemicals listed below in fire risks producing toxic vapors. Only perform flame tests outdoors with proper ventilation.
Sadly my photography skills are poor so I don't have great pictures of the actual colored flames yet. I plan to take some in the future.
For help sourcing the chemicals listed here, check out this page.
I have so far only experimented with copper compounds. Copper salts color flames blue-green when put in fire.
Copper (II) Chloride (CuCl2):
I was able to produce copper chloride by reacting some small copper peices in concentrated hydrochloric acid. The copper is lower on the metal reactivity series than hydrogen, so it won't react in a single placement reaction. Instead, the reaction here happens because the solution is open to the air. Air trapped in the solution oxidizes the copper to copper (II) oxide (CuO), and then the 2HCl + CuO -> CuCl2 + H2O. The reaction took a couple days, and was sped up by leaving the flask of solution out in the sun to heat.
In order to increase the amount of oxygen in the solution, I also added some hydrogen peroxide (H2O2) which decomposes into H2O and O2. The bubbles from the H2O2 can be seen in the below picture. Note the bright green color of the solution.
Copper chloride solution in an Erlenmeyer flask with a light underneath it.
The CuCl2 solution was then mixed with some denatured alcohol and put in a spray bottle. I then could produce bright spurts of green flames by spraying a puddle of the solution on the pavement, lighting it on fire, and then spritzing it with more solution.
Copper (II) Acetate (Cu(CH3COO)2)
Copper (II) acetate can be made is a similar manner to copper (II) chloride. The copper reacts in acetic acid (vinegar). The single replacement of H+ ions by Cu2+ ions is not energetically favorable due to copper being lower on the reactivity series than hydrogen, but exposure to air and hydrogen peroxide can oxidize the copper and allow it react with the acid. Since vinegar is only 5% concentration and acetic acid is a weak acid, this reaction is much slower than that with HCl; in my case I just left it for a couple weeks in a jar and came back to it later. Heat will likely make it proceed faster, however. The copper (II) acetate was then dried and left a pretty dark blue hydrated salt. The solution of this will still burn green, but the blue copper (II) acetate makes an interesting contrast to the green copper (II) chloride.
Blue-green copper acetate crystals on a paper.