said by Juggernaut:
True fog lights have yellow lenses for high contrast. They are rare. I would think you are talking about driving lights.
First off, a good set are as bright as high beams, or brighter depending on the bulbs that are in them. Make sure they are aimed properly.
Wire the relay so that when you turn on the high beams, they go on as well. You can, and will blind people if they are on at all times.
Fog lights come in clear lens and yellow lens versions. Some folks swear that the yellow light lets them see better, others don't notice much difference. The main thing that determines a real fog light is the beam pattern. Good fog lights will have a wide beam, with a very sharp upper cutoff to prevent light from shining up into the fog. You want the light to stay low and shine under the fog as much as possible. When you shine them on a wall you'll see a sharp line across the wall that delineates between light and dark.
(From www.rallylights.com: A Fog Pattern is very wide and very flat. Maybe 170 Degrees wide and only a couple feet high. It's only good for a couple hundred feet by design. It's purpose is to light up the road in bad weather without reflecting light off the snow/fog/rain back into the driver's eyes. A clear fog lamp is often used to fill in the area the low beams don't get to light up close to the car and the sides of the road. Some people like amber fog lamps because they claim that the yellow light causes less eye strain in bad weather and gives you a little more contrast.)
As for wiring them to the high beams, that will render your fog lights completely useless. Turn on your high beams in the fog (or even in heavy snow) and all you do is shine light on the fog (or snow), illuminating the fog or snow (referred to as back-scatter) and making it nearly impossible to see where you're going. Driving lights can be wired to your high beam circuit but fogs should be used only with the low beams, or alone. Fogs keep the light down at the road.
Here's a link to the lights that Hella offers: »www.myhellalights.com/index.php/···n-lamps/
Most of their fog lights are clear but they do offer some versions in yellow.
More info and relay wiring diagram: »coloradok5.com/wiringlights.shtml