diff --git a/src/thoughtsync/colors.py b/src/thoughtsync/colors.py
new file mode 100644
index 0000000..3158981
--- /dev/null
+++ b/src/thoughtsync/colors.py
@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from .models.note import NOTE_COLORS
+
+# Notes and labels share one colour palette (their sets were identical). NOTE_COLORS
+# is the canonical vocabulary (defined on the model); this module is the single home
+# for the "clamp to the palette" normalizer so notes.py, labels.py and sync.py stop
+# each carrying their own copy.
+
+__all__ = ["NOTE_COLORS", "normalize_color"]
+
+
+def normalize_color(color: object) -> str:
+    """Return `color` if it's a known palette key, else the default. One definition
+    for both notes and labels."""
+    return color if color in NOTE_COLORS else "default"
diff --git a/src/thoughtsync/common.py b/src/thoughtsync/common.py
index 4c0f074..2a4b6f5 100644
--- a/src/thoughtsync/common.py
+++ b/src/thoughtsync/common.py
@@ -7,6 +7,12 @@ from datetime import datetime
 # definition instead of a near-identical copy per module.
 
 
+def iso(dt: datetime | None) -> str | None:
+    """A tz-aware datetime as an ISO-8601 string, or None. Collapses the
+    `x.isoformat() if x else None` idiom repeated across every serializer."""
+    return dt.isoformat() if dt else None
+
+
 def parse_dt(raw: object) -> datetime | None:
     """Parse an ISO-8601 timestamp (accepting a trailing 'Z' for UTC).
 
diff --git a/src/thoughtsync/models/note.py b/src/thoughtsync/models/note.py
index fb1bc00..bc5576f 100644
--- a/src/thoughtsync/models/note.py
+++ b/src/thoughtsync/models/note.py
@@ -8,6 +8,7 @@ from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
 from . import Base
+from ..common import iso
 
 # The Keep-style palette. Stored as a key string, so the actual tints live in the
 # frontend and can change without a schema migration.
@@ -82,8 +83,8 @@ class Note(Base):
             "pinned": self.pinned,
             "archived": self.archived,
             "trashed": self.deleted_at is not None,
-            "remind_at": self.remind_at.isoformat() if self.remind_at else None,
+            "remind_at": iso(self.remind_at),
             "recurrence": self.recurrence,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
-            "updated_at": self.updated_at.isoformat() if self.updated_at else None,
+            "created_at": iso(self.created_at),
+            "updated_at": iso(self.updated_at),
         }
diff --git a/src/thoughtsync/notes.py b/src/thoughtsync/notes.py
index 0e73a40..9c62a0f 100644
--- a/src/thoughtsync/notes.py
+++ b/src/thoughtsync/notes.py
@@ -16,14 +16,15 @@ from sqlalchemy import case, delete, func, literal_column, select
 
 from .acl import visible_to_user
 from .auth import login_required
-from .common import coerce_bool, parse_dt
+from .colors import NOTE_COLORS, normalize_color
+from .common import coerce_bool, iso, parse_dt
 from .config import Config
 from .db import session_scope
 from .responses import json_error, not_found, parse_uuid
 from .settings import get_setting
 from .unfurl import UnfurlError, unfurl
 from .models.label import Label, NoteLabel
-from .models.note import NOTE_COLORS, Note
+from .models.note import Note
 from .models.note_attachment import NoteAttachment
 from .models.note_item import NoteItem
 from .models.note_link import NoteLink
@@ -101,10 +102,6 @@ def parse_list_items(raw: object) -> list[str]:
     return [s.strip() for s in raw if isinstance(s, str) and s.strip()]
 
 
-def normalize_color(color: object) -> str:
-    return color if color in NOTE_COLORS else "default"
-
-
 def apply_filter(stmt, filter_name: str):
     """Narrow a notes query to one board view."""
     if filter_name == "archived":
@@ -1143,7 +1140,7 @@ def _serialize_revision(rev: NoteRevision) -> dict:
         "id": str(rev.id),
         "title": rev.title,
         "body": rev.body,
-        "created_at": rev.created_at.isoformat() if rev.created_at else None,
+        "created_at": iso(rev.created_at),
     }