Why is the "green" comet C/2014 Q2 (Lovejoy) red in this picture?
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 &mu;m bands, it only detects in the 3.4 and 4.6 &mu;m bands.
The comet looks red in the false-color infrared image because, relative to the 4.6 &mu;m "W2" band, it emits less in the 3.4 &mu;m "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 &mu;m band detection to a combination of reflected sunlight and thermal radiation from dust, 4.26 &mu;m emission from CO2, and a little 4.67 &mu;m 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.