Gravity field

A Gravity Wall is a special type of wall that appears in Super Mario Galaxy and Super Mario Galaxy 2. It has the power to control the direction that Gravity pulls Mario within its expanse. The Gravity Walls contain arrows to show the player the gravity direction, as well colors: blue (arrows pointing down), red (arrows pointing up), green (arrows pointing the right), grey and purple (in Super Mario Galaxy and Super Mario Galaxy 2, respectively, arrows pointing to the left). The effect of Gravity Walls is such that as soon as Mario or Luigi enters the area with the painted arrows and colored background, either character is immediately pulled towards the object that the arrows are pointing to. The direction that the arrows are pointing on the wall (and therefore the direction of gravity itself) can sometimes be changed if the player finds a Gravity Arrow and Spins it or hits it with a Star Bit. Gravity Walls appear in many levels, the most prominent of which is Bowser's Dark Matter Plant, where they act as the main obstacle of the level. The Rightside Down Galaxy and the Upside Dizzy Galaxy feature Gravity Walls as their main elements.

Trivia

 * Gravity Walls only ever appear in sections of Super Mario Galaxy and Super Mario Galaxy 2 that are played in 2-D sidescroller format, as they would essentially be ineffective otherwise.
 * Though not a Gravity Wall, the atmospheric background of the Flipsville Galaxy acts somewhat like one, as it features multiple arrows which reflect the current flow of gravity.