Porosity refers to how your hair is able to absorb and hold moisture. It is affected by the flexible outer hair layer called the cuticle, which determines how easily moisture and oils pass in and out of your hair. For most, porosity is genetic, but it can also be affected by external factors such as exposure, heat treatments and chemical processing. Knowing your hair's porosity can help you choose which type of color is right for you. Depending on your porosity, the type of hair color you choose may end up being more permanent and last longer than you want it to.
For example: Have you ever spilled red wine on a white marble countertop that has not been sealed? It leaves a stain that will not come out no matter how many times you scrub. If that counter was sealed before the spill, it would not have stained because it would no longer be porous.
Just like the counter, the condition your hair is in determines the porosity. If your hair is in good condition, a temporary color will remain temporary (6-8 shampoos). If your hair is damaged and more porous, temporary color can be more like a permanent color lasting 4-6 weeks, thus becoming a stain.