Attack Types

In the battle room, bullets of varying color represent monster attacks. These colors are the attack types. Each attack type corresponds to one of the SOULS of the eight humans, or the SOULs of monsters.

White Attack
White attacks hurt the protagonist if their SOUL collides with them. The only way to counter white attacks is by dodging them.

White is associated with monster souls.

Gray Attack
Gray attacks deal no damage and are used for aesthetic purposes, such as spelling out messages in the Bullet Board. Napstablook introduces this attack type; they spell out "REALLY NOT FEELIN UP TO IT RIGHT NOW. SORRY." One of Ice Cap's attacks also features gray attacks (in addition to red and white); Mettaton EX uses a gray attack to spell out "Happy Breaktime!".

Gray is not associated with a soul color, and may simply reference doing nothing in battle.

Red Attack
Red attacks do not deal damage but are used to warn of upcoming attacks. The warning can take on several forms, such as an attack itself flashing red (as used by Ice Cap, Mad Dummy, and Undyne), a simple red outline bordering the area of the attack (as used by Photoshop Flowey and Sans), or a red rectangle with an exclamation point in the center and a flashing sound effect (as used by Mettaton EX, Asgore, and Asriel). Ice Cap is the first monster to introduce red attacks.

Red is associated with the the protagonist's and the fallen human's SOUL, which may imply that the warnings are protecting the protagonist's SOUL.

Green Attack
Green attacks heal the protagonist, progress the battle in some way, or both. Some enemies require the protagonist's SOUL to collect green attacks so that the protagonist can spare them. Vegetoid introduces this attack type, and it is also used by Woshua, Parsnik, Whimsalot, the Royal Guards, and the human SOULs during the Photoshop Flowey battle. Tsunderplane also uses the green attack, but its green attack is the only one that does not heal the protagonist.

Green is the attack type and color of the green SOUL and is associated with Shield Mode and kindness.

Light Blue Attack
Light blue attacks, also known as blue attacks, do not deal damage if the protagonist's SOUL remains still. Blue attacks are the opposite of orange attacks. Doggo introduces blue attacks and the attack type is used later on by Lesser Dog, Greater Dog, Dogamy and Dogaressa, Gyftrot, Woshua, Papyrus, So Sorry, Mettaton, Asgore, Sans, and the colored laser beams found in Hotland.

After the Undertale patch, the blue attacks use a slightly different shade of blue so it can be easier to see for people who are colorblind.

Light blue is the color of the light blue SOUL and is associated with patience.

Orange Attack
Orange Attacks do not deal damage if the protagonist's SOUL is moving. Orange attacks are the opposite of light blue attacks. The attack type first appears with the colored laser beams found in Hotland and is also used in the fights against Pyrope, So Sorry, and Asgore.

Orange is the attack type and color of the orange SOUL and is associated with bravery.

KARMA
All of Sans's attacks use KARMA (KR). This effect drops invincibility frames to zero (allowing the opponent's attacks to potentially hit every frame or 30 times per second). It also applies a 'poison' effect to all attacks: getting hit by any attack for even one frame will not only inflict the one damage but also increase KARMA by some amount (depending on the attack). Whenever KARMA is above zero, it will eventually drain back down to zero in increments of one, with each drain decreasing the player's HP by one. While KARMA is above zero, it appears as a purple health bar overlaid on top of the usual yellow health bar and also causes the player's HP to be displayed in purple text instead of white.

Getting hit by multiple attacks in a row will cause the increases in KARMA to stack up. KARMA cannot exceed 40 and also cannot cause the player's HP to fall below one. The more KARMA damage is built up, the faster it will drain itself and the player's HP with it.

For example, getting hit by a certain attack for one frame will inflict one damage but may also build up KARMA by nine. This means that in the seconds after the assault, the player will continue to take one damage nine more times until KARMA is drained back to zero. If the player gets hit again by the same attack for one frame, KARMA will again increase by nine (even if it has not finished draining yet), causing the player to take one damage another ten times.

The items that increase INV, the Torn Notebook and Cloudy Glasses, will grant small fractions of INV during the Sans battle, with both items together increasing INV from zero to one-third (allowing the player to obtain a single frame of INV every three frames, such that Sans's attacks can hit 'only' up to 23 times per second instead of 30). Uniquely, they will also reduce the rate at which KARMA drains.

Purple is the mode and color of the purple SOUL and is associated with Trap Mode and perseverance, but it is unknown if this is related to KARMA.