How does salt keep clothes color safe?

Question

I add salt to the laundry to keep the colors from running on my clothes but why does this work ?

Answer 1

Salt is used in most dyeing operations to drive the dye onto the fabric when dyeing cotton or rayon.  It's just the chemistry of the dyestuffs used on these materials.  Polyester on the other hand uses acid.  The dyes for cotton and rayon and actually more attracted to the water than they are the fabric which is why they tend to bleed more when washed, thus the need for the salt.  

Answer 2

Salt can be useful in dyeing, by encouraging the fiber not to repel the dye, or by making the dye less soluble,

Traditionally, cotton is dyed using water-soluble dyes, but these compounds don't naturally adhere well to cotton, so large amounts of salt must also be added to the mix to make the dye less soluble and better at adhering. The amount of salt needed sometimes approaches ratios of 1-to-1 by weight of the fabric. Large amounts of water are also needed -- it takes eight gallons of water to dye one pound of fabric.

Solubility is simply a measure of how much of solute dissolves (i.e. ionizes, dissociates- like NaCl -> Na+ + Cl-)

in how much of solvent.