This question on Scifi.se: Why does warp plasma burn green? mentions a fictional type of plasma called 'Warp Plasma' that is capable of igniting and when it does it sprays plasma flames/gas out of pipes as if it were oil or some other form of fuel.
Wikipedia says that the sun consists of hot plasma interwoven with magnetic fields. I couldn't find anything about plasma ignition (This is what I originally thought might be a reason why stars are so bright).
Is plasma capable of igniting?
Plasma is a kind of matter very similar to gas in which atoms have been ionized. It is very hot and lets off radiation (also in the visible spectrum) due to recombination so it might already be considered flame. Thus it isn't clear what "igniting plasma" really means.
In cold plasma where only a small fraction (few percent) of atoms have been ionized you can have normal chemical reactions (aided by plasma's high temperature). Example of such a process occurs in plasma created by lightening and produces ozone (O3). Thus mixing in the right chemicals can result in oxidation occurring within plasma which could be considered burning. For example plasma made up of element which easily oxidizes could be affected by mixing in gas oxygen. I guess it could be considered ignition, but it seems a stretch relative to the ordinary meaning of the word since even cold plasma is already very hot (few thousands degrees Celsius) and causing oxidation to occur inside such plasma would not add a lot of heat (in relative terms).
As for the color, careful engineering can produce plasma where recombination energy is such that light of a given color is emitted.
So in short plasma does not burn green in a chemical sense.
It can rather be quite colorful as can be seen in this picture from the remains of a supernova explosion:
If you have an oxygen plasma for example this can be used to "burn" organic compounds as is often used in the semiconductor industry to clean wafers. The plasma in this case is already at such high energies that it will immediately react with anything that can be oxidized. So you cannot ignite it even further, it is already quite reactive.
If the plasma itself should burn we have to define burning differently. Instead of a chemical reaction nuclear reactions are also possible. To ignite such a nuclear reaction in a plasma you need quite special conditions that exist for example in the sun. These processes are quite different from a chemical burning though and involve reactions at much higher energies.
Ignition in what sense? Chemical energy is out of the question, because plasma by definition are mostly ions unbound of their electrons, which is needed for chemical binding.
Nuclear fusion ignition, well, that is the whole point behind nuclear fusion research. So if someone finds how to "ignite" plasma, it will revolutionize the world, the inventor will become filthy rich, or, worst case, receive a Nobel prize.
There is an independent group trying to do focus plasma fusion with something that resembles huge spark plugs. I have no idea how likely it is that they will find anything interesting soon.
It is interesting because your question meets the research topic of an active grop of plasma physics researchers: it is called Plasma assisted ignition (PAI) and combustion (PAC).
Two examples.
The radicals mentionned by Alexander (when saying "plasmas are at such high energies...") are chemical species, created by the plasma (example: O atoms in an air - N2:02 - plasma, chops of organic compounds with very big reaction constants in air:hydrocarbon mixtures etc), which are very short lived (very short lifetime, because they react very fast with other species that can be found in the mixture under study), and normaly "could not exist in big relative concentrations" in the mixture without the action of the plasma, or of very high temperatures. These radicals, which need plasma to exist massively in the mixture at room temperature, react very fast, and with very low activation barriers. The activation barrier is for example what prevents coal to burn at room temperature: you have to heat it first, even locally with the flame of a match, so that reagents can consume their kinetic energy to start the reaction process, and the reaction chain that will lead to sustained combustion. With plasmas you no longer need an activation energy, or temperature: thus, while combustion starts after hundreds of microseconds and sometimes milliseconds in the case of successful car engine spark ignition, with a proper high voltage nanosecond discharge you can decrease this delay by one or two orders of magnitude, just because the activation factor is not only heat (like, mostly, in sparks), but the dissemination of radicals at a relatively high concentration, high enough to start the combustion at a much lower temperature, sometimes room temperature - or less.
The other application of plasmas, Plasma assisted combustion, shows up especially in unproper mixtures: lean or over-rich mixtures of gas and air, for example. Without plasma, gas can burn with air within some margin of their relative concentration: say 40-60% (don't remember the correct numbers). With the aid of a small, low-energy plasma you can extend this to much more, like 20-70, again because of all the radicals you create. You can stabilize the flame in the burner, although it would have vanished otherwise.
So, this to say, that indeed, in addition to any nuclear consideration, plasmas do "ignite" reactive mixtures in which they would be created, and not only with the heat produced by spark plugs!
A normal flame is already a plasma (Darpa is currently working on a project (Instant Fire Suppression) to control/manipulate flame by using all the plasma techniques ). Now to get a green flame, you just need to add cupper in it.
