The porosity of hair is its ability to retain water.
Some hair types are naturally more porous and act like a sponge with products or water. A contributor to hair becoming more porous are chemicals or excessive heat. Find out what could be causing the damage to your hair.
Porous hair will “suck in colour” and in the same breathe releases what it is unable to hold.
To improve the porosity use hydrating products leaving them in whenever you can. Limit drying or hot ironing hair to a minimum.