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Note that the article mentions a Thermal imaging camera, not just an infrared camera. Both are not exactly identical. A typical infrared camera works only just beyond the wavelength of red light ( called near infra-red radiation). All the sensors used in our mobiles, and digital cameras are sensitive to this wavelength ( you can just point a mobile phone camera at the TV remote when changing channels, and you can see the LED's blinking, which is invisible to the eye). These kind of cameras are useful to detect heat, but only in clear atmospheres. It'll be practically as useless as a normal camera in very dense fog. But one advantage is that while colors will be slightly jumbled up, with appropriate software, it is possible to distinguish between different colors.
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Thermal cameras will work based on minute temperature differences between points in it's field of view. That way, they can see through very dense fog also. If the sensor is cooled, then these cameras can see things far away even in zero visibility conditions. Since the signal light that is switched on will always have a higher temperature than its surroundings ( also signal lights are focused and don't spread out that fast), if the LP/ALP are familiar with the placement of different colors on the signal box, it is technically possible to identify which color is switched on.
With a proper combination of cooled thermal sensors and cooled CMOS/CCD infrared sensors, it is possible to differentiate between colors with on-board software and display it to the driver. The colors will not be exactly what the eye sees, but a differentiation is possible.