Flames are composed of burning gases. These gases may come from burning materials such as wood, or they may come pre-prepared for burning, such as methane, propane, and butane which are used for cooking food and heating houses by using furnaces. All flames I know of emit light of some kind or another, and the color and amount of light that's emitted depends on the chemical composition of the gas that's burning and on the temperature of the flame. The colors are consequences of the discrete energy levels available to electrons in orbit around the atoms making up the gas. When the materials oxidize, chemical bonds break and new ones form as the atoms rearrange themselves to form new molecules, releasing energy. Electrons change from one energy level to another, emitting light when they fall from a higher energy level to a lower one. Discrete energy levels means you get large amounts of light with definite colors.
Some articles on this Questions and Answers site refer to "luminous" flames as yellow, and non-luminous ones as the blue ones. But flames come in many more colors than these. Yellow flames can be caused by small amounts of sodium in the burning material. Adding a small amount of table salt to a blue gas flame will turn the whole thing bright yellow, while not changing the heat production at all. Woods contain sodium, and natural gas is usually delivered without any. Many yellow flames come from little pieces of soot which don't heat up quite enough to contribute to the blue part of the spectrum. Barium compounds produce greenish flames, strontium and lithium salts make bright reds, calcium salts make orange flames. And not all blue flames are dim -- adding copper compounds to fireworks makes very bright blue flames, which I would call "luminous."
Some flames which are not very luminous are also not very hot, such as the burning of ethyl alcohol or isopropyl alcohol. Some are quite hot, such as the burning of acetylene.
I do not know of any flames that are colorless. Every hot object will glow, even if it is not burning, and the color depends on how hot it is.
Tom (mods /mw)
(republished on 07/24/06)