It’s common to wonder whether or not Fog lights are useful. They certainly are. They are useful when driving in poor visibility conditions caused by rain, fog, snow, or even an excessive amount of dust in the air. They should nevertheless be used only is such conditions. In some places, it's actually illegal to drive with just your fog lamps on even if your main beam headlights are producing glare. You must use headlights only when visibility is seriously compromised bringing it down to less than 100 metres (328 feet). That’s when you use fog lamps only to be switch off when visibility improves.
Good and legally permissible fog lamps may produce white or Selective Yellow light—it is the beam pattern and not the light colour that defines a fog lamp. Most use tungsten-halogen bulbs though there are some legitimate (and many illegitimate) LED fog lights beginning to appear. Yellow light supposedly penetrates further because of its long wavelength, but it's still much smaller than fog particles. The scattering effects of fog are essentially independent of wavelength. Any colour would do just as well. So does white, and there's no reason to put on a filter.
Not all vehicles come installed with the ideal light for fog and most people tend to install them as a personal safety accessory. Fog lamps have a unique narrow and flat beam and are placed near the bumper. They have a separate switch from headlights enabling you to use either, depending upon the weather conditions. Selection of the appropriate fog lamps involves colour temperature and colour brightness measured in Kelvins and Lumen respectively. Remember that the colour temperature and visibility are inversely proportionate. This means, the higher a colour’s temperature, the less visible it is. Fog light bulbs have been traditionally yellow. Yellow light has a colour temperature of 3000 Kelvins making it suitable for poor weather conditions as it is extremely visible. Yellow light has been found to be the best in poor visibility conditions as compared to other lights and is, therefore, your best pick.
The next and most important consideration is local legislation. Most city laws specify that fog lamps should be white or amber. Oregon Law, for instance, states that they shall either be white or yellow. Lights such as blue are prohibited. Additionally, they should be turned off when one is within 500 feet of an oncoming vehicle. A distance of 350 feet is specified when one is following another vehicle. Similarly, Washington City laws specify white and amber as the acceptable light colours. Washington laws also prohibit the use of fog lamps if the low-beam headlights are not in use. Please, therefore, confirm what’s legally acceptable in your city before making your purchase.
What you also need to know is whether to replace the entire assembly or a simple LED upgrade is sufficient. If for instance, you were involved in a crash and your front or rear assemblies were totally busted, you would probably need to replace the entire assembly and the casing. Considering a switch to LED bulbs? Although costlier than halogens, LEDs save you long term costs. A pair of new halogens would last close to 24 months whereas LEDs run for years, significantly saving you replacement costs. They also consume a lot less power than halogens.