diff --git a/pdfannots/cli.py b/pdfannots/cli.py
index 05f8a24..806b108 100644
--- a/pdfannots/cli.py
+++ b/pdfannots/cli.py
@@ -11,14 +11,14 @@
 from .printer.json import JsonPrinter
 
 
-MD_FORMAT_ARGS = [
+MD_FORMAT_ARGS = frozenset([
     'condense',
     'group_highlights_by_color',
     'print_filename',
     'sections',
     'use_page_labels',
     'wrap_column',
-]
+])
 """Names of arguments passed to the markdown printer."""
 
 
diff --git a/pdfannots/printer/json.py b/pdfannots/printer/json.py
index 7fc2bde..fade3ec 100644
--- a/pdfannots/printer/json.py
+++ b/pdfannots/printer/json.py
@@ -23,7 +23,7 @@ def annot_to_dict(
         "contents": annot.contents,
         "author": annot.author,
         "created": annot.created.strftime('%Y-%m-%dT%H:%M:%S') if annot.created else None,
-        "color": annot.color.ashex() if annot.color else None
+        "color": ('#' + annot.color.ashex()) if annot.color else None
     }
 
     # Remove keys with None values in nested dictionary and return
diff --git a/pdfannots/printer/markdown.py b/pdfannots/printer/markdown.py
index 0b30c28..48c1126 100644
--- a/pdfannots/printer/markdown.py
+++ b/pdfannots/printer/markdown.py
@@ -85,16 +85,14 @@ class MarkdownPrinter(Printer):
     def __init__(
         self,
         *,
-        condense: bool = True,                    # Permit use of the condensed format
-        print_filename: bool = False,             # Whether to print file names
-        group_highlights_by_color: bool = False,  # Whether to group highlights by color
-        remove_hyphens: bool = True,              # Whether to remove hyphens across a line break
-        use_page_labels: bool = True,             # Whether to use page labels
-        wrap_column: typ.Optional[int] = None,    # Column at which output is word-wrapped
-        **kwargs: typ.Any                         # Other args, ignored
+        condense: bool = True,                  # Permit use of the condensed format
+        print_filename: bool = False,           # Whether to print file names
+        remove_hyphens: bool = True,            # Whether to remove hyphens across a line break
+        use_page_labels: bool = True,           # Whether to use page labels
+        wrap_column: typ.Optional[int] = None,  # Column at which output is word-wrapped
+        **kwargs: typ.Any                       # Other args, ignored
     ) -> None:
         self.print_filename = print_filename
-        self.group_highlights_by_color = group_highlights_by_color
         self.remove_hyphens = remove_hyphens
         self.use_page_labels = use_page_labels
         self.wrap_column = wrap_column
@@ -279,10 +277,12 @@ def __init__(
         self,
         *,
         sections: typ.Sequence[str] = ALL_SECTIONS,  # controls the order of sections output
+        group_highlights_by_color: bool = False,     # Whether to group highlights by color
         **kwargs: typ.Any                            # other args -- see superclass
     ) -> None:
         super().__init__(**kwargs)
         self.sections = sections
+        self.group_highlights_by_color = group_highlights_by_color
         self._fmt_header_called: bool
 
     def emit_body(
@@ -312,43 +312,34 @@ def fmt_header(name: str, level: int = 2) -> str:
         # Partition annotations into nits, comments, and highlights.
         nits = []
         comments = []
-        # Create a defaultdict to hold grouped highlights by color.
-        highlights_by_color: typ.DefaultDict[
-            typ.Union[RGB, str],
-            typ.List[Annotation]
-        ] = defaultdict(list)
-        # Create just a normal list for highlights when the defaultdict above is not needed
-        highlights = []
+        highlights = []  # When grouping by color, this holds only the undefined annotations
+        highlights_by_color: typ.DefaultDict[typ.RGB, typ.List[Annotation]] = defaultdict(list)
+
         for a in document.iter_annots():
             if a.subtype in self.ANNOT_NITS:
                 nits.append(a)
             elif a.contents:
                 comments.append(a)
             elif a.subtype == AnnotationType.Highlight:
-                if self.group_highlights_by_color:
-                    if a.color:
-                        color: typ.Union[RGB, str] = a.color
-                    else:
-                        color = "undefined"
-                    highlights_by_color[color].append(a)
+                if self.group_highlights_by_color and a.color:
+                    highlights_by_color[a.color].append(a)
                 else:
                     highlights.append(a)
 
         for secname in self.sections:
-            if (
-                self.group_highlights_by_color
-                and highlights_by_color
-                and secname == 'highlights'
-            ):
+            if (highlights or highlights_by_color) and secname == 'highlights':
                 yield fmt_header("Highlights")
+
                 for color, annots in highlights_by_color.items():
-                    yield fmt_header(f"Color: {color}", level=3)
+                    yield fmt_header(f"Color: {color.ashex()}", level=3)
                     for a in annots:
                         yield self.format_annot(a, document)
-            else:
-                if highlights and secname == 'highlights':
-                    for a in highlights:
-                        yield self.format_annot(a, document)
+
+                if highlights and self.group_highlights_by_color:
+                    yield fmt_header("Color: undefined", level=3)
+
+                for a in highlights:
+                    yield self.format_annot(a, document)
 
             if comments and secname == 'comments':
                 yield fmt_header("Detailed comments")
diff --git a/pdfannots/types.py b/pdfannots/types.py
index f2d5165..a7b7531 100644
--- a/pdfannots/types.py
+++ b/pdfannots/types.py
@@ -493,10 +493,11 @@ class RGB(typ.NamedTuple):
     blue: float
 
     def ashex(self) -> str:
+        "Return a 6-character string representing the 24-bit hex code for this colour."
         red_hex = format(int(self.red * 255), '02x')
         green_hex = format(int(self.green * 255), '02x')
         blue_hex = format(int(self.blue * 255), '02x')
-        return f"#{str(red_hex)}{str(green_hex)}{(blue_hex)}"
+        return red_hex + green_hex + blue_hex
 
     def __str__(self) -> str:
-        return self.ashex()
+        return f"RGB({self.ashex()})"