Two examples.
A silvery beam of pale light shines down in a 5-foot-radius, 40-foot-high cylinder centered on a point within range. Until the spell ends, dim light fills the cylinder.
—Moonbeam, Player's Handbook, pg. 261
You create up to four torch-sized lights within range, […] each light sheds dim light in a 10-foot radius.
—Dancing Lights, Player's Handbook, pg. 230
Now consider that each of these spells are cast on a bright day, obviously covered by Bright Light, as the game understands it.
- Do these spells change the Bright Light within the area affected by them into Dim Light?
- Is the behavior different between Moonbeam ("Dim Light fills the cylinder") and Dancing Lights ("each light sheds dim light"), or do both of them behave the same?
Best Answer
Spells override Bright Light with Dim Light if the spell description says that it does.
Spells do what they say.
Dancing Lights states that "you create up to four torch-sized lights within range."
Moonbeam states that "until the spell ends, dim light fills the cylinder."
The light sources shed dim light, but nothing says this is a special type of dim light, or that the light is part of the spell's effects.
Moonbeam specifically indicates that the area of the spell is filled with dim light.
If the light from Moonbeam were overridden by the area being normally filled with bright light, this would make the spell's area invisible which seems counter to the intent.
So to answer the questions about your examples directly:
Moonbeam does, Dancing Lights does not.
The behavior is different between the two spells.