In 2010 I changed my seat belts from black to red in my 993 and Boxster. It was a fairly easy DIY. But, I took some heat on another forum as some thought it was a safety issue. IMHO, it isn't. The material is made by a company that makes seat belts and the joints were sewn by a cobbler that used a strong thread and over stitched it.
Searching the web I found
http://www.gotbelts.com/ . They sell 29 colors of seat belt webbing by the yard. They also sell the small seat belt stop buttons.
The seats belts are like window shades. You pull them all the way out and they are simply pulled through the center of the roller and held in place with a small plastic dowel. Once I pulled it out I held it open with a small plastic clamp.
I measured the webbing out and found that there was nine feet of webbing on each belt. I ordered 6 yards of red seat belt webbing and two stop buttons. Total cost shipped was $48.84 in 2010. Checking their site today they are charging $46.50 for 6 yards and two stop buttons plus shipping.
I then cut the new seat belt webbing, cut off the old webbing and put everything back together with the red webbing. I used safety pins to keep the ends of the webbing in place for the trip to the cobbler. The only warning is to make sure the cobbler know to keep the seam on the dowel end small. Take the one you cut off into him and have him replicate it. If it is too thick or too long you will have issues with the reassembly. It isn't a bad idea to take the other end in with you too so he can see how the factory sewed it.
The cobbler charged $20 for sewing four joints.
Photo of the plastic doweling pulled part way out.
Photo is the seat belt assembly with the belt pulled out and clamped off.
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Below is a photo of the flame red on the right on the red on the left. The red is very close to OEM; the flame red is brighter
These are the colors that Go Belts offer.