GW170817 optical and infrared light I'm reading about GW170817 in various research articles. One of them,

The X-ray counterpart to the gravitational-wave event GW170817. E. Troja et al. Nature 551, 71 (2017), arXiv:1710.05433

includes the following figure. 



However, I am not able to distinguish what the colours correspond to. Are they representative of their colour in the visible light spectrum? Which one would be associated with the infrared light?
 A: The colours in the figure you've quoted are arbitrary: they have no relation to the data being plotted, and they have only been added to distinguish the curves visually from each other, to make the figure easier to read.
The figure shows spectra of the same source as measured at different times after the event started, and the numbers next to the curves indicate the dates (i.e. 0822 is August 22) at which the data was taken. The colour coding is there (partly) to make a clearer link between each curve and its label.
As the figure caption indicates, there is only one spectrum taken in the visible range, i.e. the 0821 curve on the left, which covers the visible range $0.4\mu\mathrm m \lesssim\lambda \lesssim 0.8 \mu\mathrm m$. Everything to the right of the $0.8 \mu\mathrm m$ mark is in the infrared.
A: The wavelength determines the color, so different curves can't represent different colors if they have the same wavelength.  Instead, it appears that these curves represent spectra taken at different times.  It's important to measure the spectra at different times to figure out things like how rapidly the matter "cooled".  This can suggest what types of nuclear reactions were taking place, or how close that matter might be to the merged star, for example.
The clue comes from the legend, which says that the spectra were taken starting August 21 — which is 0821.  More specifically, it says "The colour-coded numbers indicate the epoch of each spectrum."  Each time you measure the spectrum, you have to watch the source for a little while.  During that relatively short time, you assume the spectrum is basically constant, but you come back a somewhat longer time later to check again.  Each of the short time spans is called an "epoch".
Different telescopes cover different parts of the spectrum, which is the other reason these curves all look different.  Hubble alone has multiple cameras, each having multiple configurations.  In particular, it has many filters, grisms, and polarizers that can be selected.  Evidently, they used different combinations in different epochs for some reason.  The "Methods" section of that paper has some details, but doesn't note such specifics.  The camera in question is Hubble's WFC3, which has IR grisms covering the ranges 800-1150nm and 1075-1700nm.  So evidently, 0822.4 and 0828.1 were each taken with at least two exposures done very close together and combined after the fact.  Presumably 0824.8 was done with just the first grism, and 0827.0 was done with the second.
