I've have a set of Hella Rallye 500 Fogs in yellow, for a couple decades now. I've never seen a set of fog lights that could beat them during fog conditions, they cut under the fog due to fluting in the glass that cuts off the light above the fog line. Hella discontinued them in the yellow a long time ago, they make the Rallye 450 (rectangle) & Rallye 500FF in yellow, but I like the round and the FF just doesnt work the same! Mine have been thru alot having been on a few trucks, a bunch of cars and even a dirtbike!
The yellow inside on the reflector had peeled off and disintegrated quite some time ago.

Step 1:
I stripped them down to just the glass & metal, no back housing(plastic) & no bulb.
Step 2:
Put them in the oven & PreHeat the oven. Best to do this while the wife or girlfriend will be out all day!
I wasnt sure what temp would be needed, so I started low... 170*F, lowest setting on my stove.
After about 15mins, I removed them with leather gloves on.
Step 3:
Separate the lens and reflector using a flat screwdriver, twisting motion. Just a little twist each time, moving 1/2" around the light. It doesnt take much force and after a 1/2 dozen twists I started to hear the old glue give way.
Step 4:
Clean the inside of the light. Get rid of any dirt, dust or old colored film, like in mine.


Now decide how/what you want to do for treatment. I decided that I wanted to tint the reflector yellow(like Hella had done) and paint the bulb holder in the center black(its grey in real life, not black like the pic). If you decide to remove the bulb holder, mark with a Sharpie so its easy to assemble correctly. Its just 3 tabs that hold it on, gently bend those straight.
Step 5:
Tinted the reflectors yellow, with Krylon Stained Glass paint.

Step 6:
Painted the bulb holder Satin Black, with ordinary paint.
Step 7:
After the reflector was tinted, I put the lens on the reflector to check the amount of yellow color.

I wanted more color, so I tinted the inside of the glass as well.

Step 8:
Put everything back in the oven(still at 170*F) for drying, but turn the oven off and allow everything to cool down. This will help it cure into the parts and hold well. Wait several hours to a full day for the paint to fully dry before assembling.

Step 9:
PreHeat your reflectors and lenses to 170*F for a few minutes. Remove them, smear a thin & even coat of RTV where the 2 pieces meet and press together tightly.
Step 10:
After the RTV has cured, wipe a thin & even coat of RTV over where the 2 halves meet, to insure a good weather tight seal. Let cure.
Step 11:
Reattach the bulb holder, if removed. Reassemble light and back cover. Install on car. Enjoy!