SOUL modes

Soul Modes are used to change the color of Frisk's soul. As well as changing the color of the soul, it also changes the way it behaves. They correspond to the known human traits.

Red
The soul moves and behaves normally. It can move in any direction at a constant speed, and can slow down by pressing [X] or Shift. It sprouts legs when it flees from battle.

Yellow
When yellow, the SOUL is in Jump Mode and affected by gravity, much like a side-scrolling/platformer perspective. This SOUL color is used by both Catty and Brqtty in battle and is the first time the protagonist's SOUL changes color. Bratty appears to have much more control over this, being able to force gravity in all four directions rather than just straight down.

Pressing a direction opposite to the force of gravity causes the SOUL to "jump" in that direction. The height of the jump is dependent on how long the direction up is held until the maximum height is reached, which can depend on the attack. In fact, near the end of Catty's final attack, the max height is so great that holding the jump can cause the SOUL to rise beyond the bullet board, even causing it to extend upward to make room.

Yellow is the mode and color of the Yellow Human, who is also associated with ballet, dancing, hopping, jumping, twirling, rhythm, music, and integrity.

Blue
While Blue, the SOUL is in Shield Mode and cannot move, but wields a shield-like bone from Sans that protects the protagonist whichever direction is pressed. Instead of dodging a pattern, the protagonist must block arrows coming in from all four directions using this spear. This SOUL color is used by Sans, who switches the SOUL between green and red using his bone. It is impossible to flee in this mode. Sans gave the protagonist the bone in an attempt to force them to face danger head-on.

Blue is the mode and color of the Blue Human, who is also associated with Blue Attacks, shielding, frying pans, cooking, healing, and kindness.

Orange
When Orange, the SOUL is in Trap Mode and can only move left and right on a line and switch between lines using up and down. Bones move on these lines that the protagonist has to dodge, acting as a web. This SOUL color is used by Mae after she fills the Bullet Board with orange liquid on her first turn.

Orange is the mode and color of the Orange Human, who is also associated with traps, note taking, problem-solving, analysis, intellect, and perseverance.

Poison/KARMA is orange as well, but it is unclear if it is related to the soul type.

Light blue
While Light Blue, the SOUL is in Shooter Mode and is flipped upside down, so its heart's point is directed toward the foe. Pressing [Z] shoots a projectile that can destroy certain oncoming projectiles or set off bombs, as well as damage the enemy in certain situations. This SOUL color activates after the protagonist presses a button on the phone Papyrus gives them and is only used against Asriel and Asriel BETA as well as the Lost SOUL variation of Papyrus.

Light Blue is the mode and color of the Light Blue Human, who is also associated with guns, bullets, sharpshooting, accuracy, and justice.