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In this nice NYTimes 'survey article' "Our Vast Solar System and Its Many Explorers" there is a NEOWISE image of Comet C/2014 Q2 (Lovejoy). See here also.
The NASA description of the image says:
The red color is caused by the strong signal in the NEOWISE 4.6-micron wavelength detector, owing to a combination of gas and dust in the comet's coma.
So I understand that this is a false color image, and the image at 4.6-micron is overlaid using red. So I should really ask: why is Comet C/2014 Q2 (Lovejoy) (and probably many) comet actually bright at 4.6 microns "… owing to a combination of gas and dust in the comet's coma." Is the gas fluorescing and the dust scattering? Or is this thermal radiation from the dust? The explanation in quotes doesn't really explain why "the red color is caused by the strong signal (around) 4.6-micron wavelength (is) owing to a combination of gas and dust in the comet's coma."
below: The more familiar green glow of this (and many) comet from here.
Mainzer et al. 2014 characterize the performance of the reactivated NEOWISE. Having run out of cryogenic coolant for the original WISE's 12 and 22 μm bands, it only detects in the 3.4 and 4.6 μm bands. The comet looks red in the false-color infrared image because, relative to the 4.6 μm "W2" band, it emits less in the 3.4 μm "W1" band than stars do. Figure 3 of the paper shows a similar image in which an asteroid also looks red.
Section 4.1 discusses comets, attributing the 4.6 μm band detection to a combination of reflected sunlight and thermal radiation from dust, 4.26 μm emission from CO2, and a little 4.67 μm emission from CO. They estimate how much is due to gas vs. dust by comparison with ground-based observations near the same times, as in Figure 17.