Planetary protection is a principle aiming to prevent biological contamination of a target body, such as Mars, or of Earth in the case of a return mission. As it is unknown if there is life on Mars, the subject of planetary protection is controversial. For the moment, as of 2020, it has been applied as a precautionary measure.
Mars colonization is probably incompatible with the concept of planetary protection as applied today. Mars settlements, no matter how well designed, will leak large amounts of air and water. Experience with the International Space station has shown that the station, despite being in vacuum, has its exterior contaminated with a number of different organisms(reference needed).