Fires can burn in oxygen. But, can they burn in halogen atmospheres (fluorine and chlorine only)? I am wondering this because combustion reactions involve oxygen and carbon and other elements. But, can they burn in fluorine and/or chlorine? If they do, what color will the flame of paper and wood be in fluorine and chlorine atmospheres respectively?
They certainly can. Put some water in the bottom of a beaker, fill the beaker with chlorine gas, and drop in bits of calcium carbide. The carbide will produce bubbles of acetylene, which will spontaneously burst into flame when they hit the chlorine.
Flame colors depend on many things, even in oxygen. In general, flames in a chlorine atmosphere will probably look slightly more greenish than flames in air or oxygen, simply because the gas itself is green. Fluorine isn't as strongly colored, and flames from combustion in fluorine will be VERY VERY BRIGHT.
Note that forming flame is not limited to the particular environment like gas containing enough of oxygen, even if we are used to that.
I remember an experiment in the high school chemistry class, where the lecturer presented oxygen being burnt in flame in atmosphere of hydrogen.
For a flame, several conditions must be met:
There is ongoing fast exothermic enough chemical reaction ( or there is provided intensive external energy like MW powered argon plasma in ICP )
Reaction must provide gas, or occur in already existing gas.
Gas must be heated by reaction enthalpy enough to form ions, radicals, excited molecules or atoms, actively emitting light by releasing excitation or recombination energy in form of visible light.
The flame occurrence depends on scenario. Oxidizing and oxidized stuff may come into contact at contact area (flammable gas output lighted to prevent gas accumulation), in whole volume (welding by acetylen - oxygen flame ), or a chemical stuff burns itself, being a mixture(black gunpowder), or containing oxidative groups ( explosives and smokeless gun powder staff )
Flame color depends on emission lines of above mentioned excited particles and there is no general answer.