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Here are the claims:

Vinegar helps “set” colors and prevents build-up of detergent residue, which contributes to color fading. [...] Pour ½ cup of salt in the wash cycle to prevent the dyes in colored fabrics from bleeding.

4 Natural Ways to Keep Colors Bright

Add a tablespoon of salt to the wash, and the chloride will help our clothing pack a colorful punch when it comes out of the dryer. The reason? Chloride found in salt actually helps to seal the color in fabrics, thus keeping it from fading. [...] If you add ½ cup of white vinegar to the rinse cycle, the liquid will freshen up your laundry and help colors maintain their intensity.

The Natural Way to Keep Fabric from Fading

Is there any evidence to support the claims?

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    I doubt it. Many of these tips date from fifty years ago or more. Even if they had some utility back then, modern dyes are more resilient.
    – TheMathemagician
    Commented Sep 20, 2016 at 11:19
  • Anecdotally, I had some red cloth that I was trying to make colour fast. Washing repeatedly with salt and with vinegar had no appreciable effect, and is another reason I am a vinegar cynic.
    – Oddthinking ♦
    Commented Aug 2, 2017 at 15:46
  • As an aside - I use both a salt water soak and added vinegar to wash to try and santiize/de-funk/de-stink laundry, especially workout clothes. It seems to have a certain amount of positive effect that just detergent does not.
    – PoloHoleSet
    Commented Aug 2, 2017 at 21:22

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Salt has been used as a mordant (dye-fixer) for ages. Packs of RIT brand dye suggest adding salt (or vinegar, for synthetics) to the dye bath to help the color set.

https://www.ritdye.com/techniques/the-basics/stovetop-technique/

OTOH, they are also using the hottest water possible, which is hard on clothes, so not for frequent use. Note also that this is for when you are dying cloth and may not be any help when washing same.

But there is at least a grain (sorry) of truth to the idea.

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  • FYI - Usually when helping a dye set, the fabric's pH and chemistry is altered so the dye can chemically bond with the fabric. Once that bonding/setting is done, a normal wash restores normal pH and the fabric is not susceptible to taking on additional, unwanted dyes. Creating a better environment for accepting dye, initially, is really unrelated to whether that dye is retained, as you correctly surmised.
    – PoloHoleSet
    Commented Aug 4, 2017 at 16:27

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