If you cast hypnotic pattern in the dark (assuming the caster has dark vision), does the spell work regardless, or does the target have to be able to actually see (with or without dark vision) for the spell to affect them?
Per the spell description, the target(s) must be able to see the patterns created by the spell. Since the spell creates swirling patterns of color, I would rule that even in complete darkness the target(s) can see it, unless their eyes are closed or otherwise obscured.
My reasoning for this is that swirling patterns of color, on their own, as a magical effect, must be light-based. Though they don't emit light in the sense that a torch does, they must be visible. The spell components here are either a glowing stick of incense or a crystal vial filled with phosphorescent material -- essentially a sparkler or a glowstick. The effect that most people are probably familiar with is that of a glowstick or sparkler leaving trails across your vision in the dark. One might even say these trails create hypnotic patterns.
It might not, because Darkvision robs you of Color vision
While LegendaryDude has an excellent answer, there isn't an inherent assumption that color means light. A red ribbon is still red in the dark.
Therefore, we can't assume that the "pattern of colors" is inherently visible, meaning it provides no illumination. Thus, a creature must see it using Darkvision. All Darkvision robs you of color vision, therefore almost all characters won't see the display.
(As two exceptions, a Warlock with the Devils Sight evocation can see through darkness perfectly, as could a creature with Truesight).
Thus, in darkness creatures with Darkvision or without sight at all are not affected by it.
Though this is a very interpretive answer, and you can come up with counter evidence.
The mistake may be in guessing.
In other cases you have to guess, because some rules are very open, but this is not one of those cases.
D&D specifies when something is light.
For example: Color Spray (There are more, but this is the best example since we are talking about Hypnotic pattern).
(...) A dazzling array of flashing, colored light springs from your hand (...).
Whether it is light or just color is relevant to determining how it interacts in Magical Darkness and Non-Magical Darkness.
If it's light = you have to look at the level of both spells. If it's not light, you do not meet the requirement in both types of darkness, since in normal, as other answers have pointed out, you see in shades of gray.
The spell speaks of colored magic that produces an effect.
If you don't see the magic, you don't have the effect.
Magic is not visible in darkness unless it is light or is specified in some way.
Thanks a lot @KorvinStarmast
I was going to put a longer comment, but as I said my English is very bad and I don't want to cause problems.
I'll leave it here using google translate.
It is surely unreadable and is very similar to the one I finally put in using simpler phrases:
The spell requires that the creature within the area see the pattern.
I don't think this mechanic is only for blind enemies (blinded state, blind vision, closing eyes...).
It seems to me that it is mainly for the rules of Vision and illumination [Pag 183 Manual "(...) The most fundamental tasks of venturing: realizing danger, finding hidden objects, hitting an enemy in eombat and targeting a spell, To name just a little, largely in the ability to see of an eharaeter. Darkness and other effects that give vision can be a major obstacle (...)"].
Thus the spell would work perfectly in bright or dim Light and would fail in Darkness whether magical or not.
- In normal darkness, an enemy with Dark Vision cannot distinguish colors, only shades of gray, so it would not affect him.
- In magical darkness it would affect those who have vision in magical darkness.
Another note is if the color pattern emits light.
The spells that emit light are very well typified in the manuals, since it is an important mechanic, I will put as an example "Color Spray" (Page 222) or "Faerie Fire" (Page 239), but it is better, if you have buy a PDF, to put the word light in the search engine.
So I understand that it is colored magic, not colored light.
Note 2: Remember that you can always change, fun comes first.