This is a page about the implications of blackbody radiation, cooling and radiative transfer in STAR WARS.
The glow seen inside the nozzle of a sublight thruster is thermal (blackbody) radiation from a hot but apparently solid surface. The colour is indicative of the temperature: red is coolest. Orange, yellow, white, blue and violent are increasingly hot. If we can use colour to estimate temperature then we can calculate the radiant power of the visible surfaces. This waste heat emission is an absolute lower limit on the total power of the engine. If the kinetic power of the ship can be separately estimated from its mass and maximum acceleration, then we can compare this with the waste thermal power of the engine nozzles as an indication of engine efficiency. All else being equal, redder colours signify greater engine efficiency.
Blackbody thermal radiation consists of photons with a range of wavelengths. The effective bredth of this wavelength range is never wide enough to produce a glow that is concentrated in green photons without significant amounts of neighbouring hues. Thus no simple, thermal engine-glow can ever appear subjectively green. Darth Maul's Sith Infiltrator [TPM], however, does appear to have green engines, at first glance. This must simply be an illusion caused by the juxtoposition of small parts that are alternative yellow (cooler) and blue (hotter) within the engine outlet. Indeed a complex grille structure is visible within these engines when the ship settles to rest and cools. The apparently pink glow of the engines of a Geonosian starfighter [AOTC] must also be due to a similar multi-temperature internal structure visible through the nozzle.
There is sometimes argument about whether asteroidal material is vaporised or merely fragmented. I think that colour can help clarify or settle such arguments.
Consider blackbody temperature. If the cloud is red then the temperature is on the order of 1000K. Some material may be in solid, but finely divided, form. If however the glow is orange, yellow or hotter then it's more likely to be beyond the melting point of metal or rock. In that case we can be sure that the visible material is liquid and/or vapour.
Consider the brightness distribution of the fireball. The presence of dark or opaque obscuring material on the fringes of the cloud indicates the presence of solid, particulate or sooty material. Solids scatter or absorb light from the incandescant gas. A line of sight directed towards the middle of the explosion passes through the thinnest section of whatever superficial dust is present. The apparent dust obscuration is minimal towards the centre of the image of a fireball. However when you look towards the fringes of the cloud (from your point of view) you're looking tangentially to the cloud surface at that point. Thus you're viewing the structure through a greater thickness of the superficial solids. The cloud appears darkened along those lines of sight. In astronomy we call this effect “limb darkening” and it is measurable in stars with dusty atmospheres.
The DS2 explosion shows this effect clearly, indicating the presence of an appreciable mass of particulates or soot (rather than just vapour) within the outer parts of the fireball. As the explosion ages and cools (by radiation and by adiabatic expansion) the dust effects become more significant. This may be the effect of solid soot condensing out of the initially vapourised gas, analogous to the condensation of snowflakes from water vapour in the Earth's atmosphere.