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.
Down here, LOVE is shared through... Little white... "friendliness pellets."
Flowey at the beginning of the game
White attacks hurt the protagonist if their SOUL collides with them. The only way to counter white attacks is by dodging them. The multicolored attacks used by Undyne, Photoshop Flowey, and Asriel Dreemurr function the same as white attacks.
White is associated with monster souls.
REALLY NOT FEELIN UP TO IT RIGHT NOW. SORRY.
Napstablook's second 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 attacks usually do no 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 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. Mad Dummy's red attacks deal damage as if they are white.
Red is associated with the protagonist's and the fallen human's SOULs, which may imply that the warnings are protecting the protagonist's SOUL.
Eat Your Greens
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, and the human SOULs during the Photoshop Flowey battle. Tsunderplane and the Royal Guards also use the green attack, but their green attacks do not heal the protagonist, unlike other enemies.
Light Blue Attack
stop signs are red. so imagine a blue stop sign instead.
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.
Light blue is the color of the light blue SOUL and is associated with patience.
O-Orange ones, um... [...] Move through those ones!
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.
Yellow Attacks seem to show some kind of off/on switch or trigger, as when Mettaton's legs in battle are shot while using Shooter Mode, the legs get a yellow outline and move out of the way, becoming dodgable.
Yellow is the attack type and color of the yellow SOUL and is associated with justice.
KARMA coursing through your veins.
Flavor text from Sans's battle
Almost all of Sans's attacks use KARMA (KR). This effect applies (to use terms of a general RPG) a "poison" effect to the attack: getting hit for even one frame inflicts not just the one damage, but also increases KARMA by some amount (depending on the attack). Whenever KARMA is above zero, it eventually drains back down to zero in increments of one, inflicting one damage with each drain. Because neither flavor text nor Sans explain KARMA in-game, it is unclear if it is linked to the lack of invulnerability during the fight which allows attacks to hit up to thirty times per second.
For example, getting hit by a certain attack for one frame inflicts one damage but may also increase KARMA by five. This means that even if the protagonist does not get hit by any more attacks, even while the menu is open, they slowly take one damage five more times, for a total of six damage (after which KARMA goes back down to zero). Getting hit by multiple attacks causes the KARMA increases to stack up, so if the protagonist gets hit by another attack, their KARMA may increase by five again (even if it has not finished draining yet), causing the protagonist to take one damage another six times.
While KARMA is above zero, it appears as a pink-purple health bar overlapping the usual yellow health bar, and also causes the protagonist's HP to be displayed in pink-purple text instead of white.
KARMA cannot exceed 40 and also cannot cause the protagonist's HP to fall below one. The more KARMA damage is built up, the faster it drains.
Sans's various attacks increase KARMA by different amounts:
- Bone vertical loop: 5
- Gaster Blaster: 10
- Single menu bone (top left): 2
- Four menu bones (bottom): 1
- Other: 6
For KARMA to stack up linearly, the protagonist must be hit by separate attacks (e.g. three Gaster Blaster hits for one frame each increases KARMA by 30, dealing an eventual total of 33 damage). If the protagonist stays in a single attack, KARMA only increases by one per frame (e.g. one Gaster Blaster hit for three frames increases KARMA by 12, dealing an eventual total of 15 damage).
KARMA takes one frame to drain if maxed out at 40, two frames to drain if between 30 and 39, five frames to drain if between 20 and 29, fifteen frames (or half a second) to drain if between 10 and 19, and thirty frames (or one second) to drain if between 1 and 9. This means that KARMA when maxed out drains halfway within 66 frames (2.2 seconds) and completely within 491 frames (~16.4 seconds).
The items that increase INV, the Torn Notebook and Cloudy Glasses, grant small fractions of INV during the Sans battle. Specifically, the Torn Notebook increases INV by 1/5, the Cloudy Glasses increase INV by 1/4, and both items together increase INV by 1/3. This means that with both items together, the protagonist obtains a single frame of INV every three frames, causing Sans's attacks to hit up to 23 times per second instead of 30. The two items also slow down the rate at which KARMA drains, increasing the frame delays to 2, 4, 8, 20, and 40 respectively when equipped together. This means that KARMA when maxed out drains halfway within 114 frames (3.8 seconds) and completely within 682 frames (~22.7 seconds).
It was intended that equipping only the Torn Notebook would give a 1 in 2 chance that KARMA drain is slowed down, and that equipping only the Cloudy Glasses would give a 2 in 3 chance, but due to a bug in the code, there is barely any slowdown when one of the two items is equipped. Equipping both items together behaves correctly.
- In patch, color of blue attacks will be changed to be more visible to colorblind some people. (This should be blue?) - Toby Fox (@tobyfox) on Twitter, January 19, 2016.
- Sans' invincibility frames - Reddit