It is likely due to the white balance. I tried the same with my soldering iron. Just as 850nm-filtered images the response was identical for all three RGB-channels when the NIR image was dominant. If the environment has a lower colour temperature and is used for the white balance it will shift the hot tip colour into blue.   Set temperature 350°C:   Set temperature 400°C:   Set temperature 450°C:   All white balanced against the not so hot tube at the bottom, reflecting some of the indoor lig