Asked by: Roger Britton, via email
Many fabric dyes are acids that form negatively charged molecules in water. The colour-absorbing sheets used in your laundry contain positively charged compounds that attract any dye molecules that leach out of coloured clothes.
Once the compounds have grabbed a dye molecule, a chemical bond forms that binds the dye to the sheet, so it won’t bleed out onto the laundry again. So the science behind it is sound, but modern dyes are much more colour-fast than they used to be, so colour-absorbing sheets are often unnecessary.