I have done a DIY seat belt color change twice; once on my 993 Turbo and on the Boxster. There is always concern about doing a DIY on such an important safety component.
I have no doubt that the seams that I had sown professionally would hold in an accident. Below is a thread I started on the DIY back in 2013.
Guards Red, Speed Yellow and Riviera Blue seat belts were an option on the 986 for $330.
DIY: Red Belts for Under $70
And, the company I bought the seat belt material from,
https://www.seatbeltplanet.com/, stocks 31 colors of webbing;
https://www.seatbeltplanet.com/shopping-cart/swatches/