SOUL Modes

In battle, the player is represented by their soul: a red heart. However, many bosses in the game can alter the color and change the way it behaves.

Red
While the heart is red, it behaves normally. It can move in any direction at a constant speed, and can slow down by holding [X]. This is the true color of Frisk's soul.

Blue
When blue, the heart is affected by gravity. Pressing a direction opposite to the force of gravity causes the heart to "jump" in that direction. The height of the jump is dependent on how long you hold the direction up until you reach the max height, which can depend on the attack. This is used by both Papyrus and Sans, and is the first time you see the heart change color. Sans appears to have much more control over this, being able to force gravity in all four directions rather than just straight down.

Green
While green, the heart cannot move. Rather, you control a shield that protects the direction you press. Instead of dodging a pattern, the player must block arrows coming in from all four directions using this shield. This is used by Undyne, who switches the heart between green and red using her spear.

Purple
When purple, the heart can only move freely left and right on a line, and switch between lines using up and down. Spiders move on these lines that the player has to dodge. This is used by Muffet after she fills the square with a purple liquid.

Yellow
While Yellow, the heart is flipped upside down so the point is directed toward the enemy. 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 happens after the player presses a button on the phone Alphys gives you, and is only used against Mettaton.