How and why should a porosity test be carried out?

The reason why a porosity test is important is because hair that is more porous will "suck up" color more quickly than less porous hair. This being said, if the hair is not evenly porous, some parts may be darker than others and vice versa. To test the porosity, simply examine the quality of the hair in your hand. If the ends are severely brittle and porous, you might want to suggest a haircut after the service. Or, to make the porosity more even, apply a developer first, leave on for five minutes (don't wash it out), and then mix the desired color with developer and apply evenly.