How do you set colors, so they don't run in the wash? Red onto white, etc. Is it salt or vinegar?

I have to new shirts that are mostly white, but have red and blue as trim. If I wash them without setting the colors 1st the color will bleed all over the white!!! Help DonnaHow do you set colors, so they don't run in the wash? Red onto white, etc. Is it salt or vinegar?
Cold water will set a color too.How do you set colors, so they don't run in the wash? Red onto white, etc. Is it salt or vinegar?
If the fabric is polyester you use vinegar, if it's cotton or rayon use salt. The majority of fabrics nowadays are one of these fabrics or a blend of poly and cotton. Cotton and rayon are the worst for bleeding because the dyes on them are actually attracted more to water than to the fabric itself.