Many fabric care instructions specify that lights and darks cannot be washed together. This instruction to separate laundry may seem onerous, but it is important if you want to take care of your fabrics. There are a number of reasons to separate lights and darks. The most important is ensuring that darker colors do not bleed onto lighter ones, causing the lights to stain and discolor in an unsightly way.
Bleeding is a major issue in fabric care. Lights and darks should always be separated when they are new, as fabric dyes will inevitably bleed at least a little bit the first time that they are washed. Jeans are particularly notorious for bleeding, but dark reds and other blues can also bleed. When darks such as blues are washed together, a small amount of bleeding isn't a major issue, since it doesn't show on the fabric. If, however, blues are washed with whites, the bleeding will stain the whites. Blues can also stain reds and other lighter colored fabrics, so some people separate their laundry into further gradations of color than just colored and white.
Fabric care instructions also vary, depending on the color. Lights, for example, can be bleached to remove stains and discoloration, while darks cannot. A light load can also be washed at a higher temperature, which may be a concern to people who are trying to sterilize laundry which has been contaminated in various ways. A dark load generally requires a cooler temperature, as this reduces the risk of bleeding, and some darks may require special detergents.
When you separate laundry into the two categories, you should also be aware that certain fabrics require special care. Some fabric, for example, should not go in the dryer, while specialty garments may require a gentle cycle or cooler water. It is a good idea to separate gentle care items to ensure that they are not accidentally damaged in a load of more sturdy laundry, and you should always check garment labels to ensure that a garment does not need to be hand washed.
If you do not have the time or inclination to separate laundry into lights and darks, you should always wash clothing on cold, to reduce the bleeding of dye. You should also use a mild, generic detergent, and a normal to gentle cycle on the washing machine. Delicate garments can be washed in pillowcases or garment bags to reduce damage, and always dry on medium to low so that garments do not shrink or fade.