diff --git a/lib/mail_mcp/tools/update_mail_message_flags_tool.rb b/lib/mail_mcp/tools/update_mail_message_flags_tool.rb
index f64b88f..db844cc 100644
--- a/lib/mail_mcp/tools/update_mail_message_flags_tool.rb
+++ b/lib/mail_mcp/tools/update_mail_message_flags_tool.rb
@@ -10,25 +10,56 @@ class UpdateMailMessageFlagsTool < Tool
       open_world_hint: true
     )
 
+    # IMAP allows only these six backslash-prefixed system flags; anything else
+    # must be sent as a keyword (a bare atom with no leading backslash).
+    SYSTEM_FLAGS = %w[Seen Answered Flagged Deleted Draft Recent].freeze
+
+    # Friendly color names map to Mozilla/Thunderbird $labelN keywords, the
+    # de-facto cross-client convention for colored flags.
+    COLOR_KEYWORDS = {
+      "red" => "$label1",
+      "orange" => "$label2",
+      "green" => "$label3",
+      "blue" => "$label4",
+      "purple" => "$label5"
+    }.freeze
+
     input_schema(
       type: "object",
       properties: {
         folder: { type: "string" },
         uid: { type: "integer" },
         add: { type: "array", items: { type: "string" },
-               description: "Flags to add, e.g. ['\\\\Seen', '\\\\Flagged']" },
+               description: "Flags to add. Settable system flags: '\\\\Seen', '\\\\Answered', " \
+                            "'\\\\Flagged', '\\\\Deleted', '\\\\Draft' (\\\\Recent is recognized " \
+                            "but cannot be set by clients). Color names " \
+                            "('red', 'orange', 'green', 'blue', 'purple') map to " \
+                            "$labelN keywords. Any other value is sent as a custom keyword." },
         remove: { type: "array", items: { type: "string" }, description: "Flags to remove" }
       },
       required: %w[folder uid]
     )
 
     def self.call(folder:, uid:, server_context:, add: [], remove: [])
-      add_flags    = add.map(&:to_sym)
-      remove_flags = remove.map(&:to_sym)
+      add_flags    = add.map { |f| normalize_flag(f) }
+      remove_flags = remove.map { |f| normalize_flag(f) }
       ImapClient.connect(server_context.imap_config) do |c|
         c.update_flags(folder: folder, uid: uid, add: add_flags, remove: remove_flags)
       end
       MCP::Tool::Response.new([{ type: "text", text: "Flags updated for message #{uid}" }])
     end
+
+    # System flags become Symbols (net/imap renders them with a leading
+    # backslash). Custom keywords stay Strings (rendered as bare atoms).
+    # Friendly color names are translated to their $labelN keyword first.
+    def self.normalize_flag(flag)
+      name = flag.to_s.delete_prefix("\\")
+      return COLOR_KEYWORDS.fetch(name.downcase) if COLOR_KEYWORDS.key?(name.downcase)
+
+      # System flags are case-insensitive per RFC 3501; normalize to the
+      # canonical capitalization so e.g. "seen" / "\\SEEN" still set :Seen.
+      system_flag = SYSTEM_FLAGS.find { |f| f.casecmp?(name) }
+      system_flag ? system_flag.to_sym : name
+    end
   end
 end
diff --git a/spec/mail_mcp/tools/update_mail_message_flags_tool_spec.rb b/spec/mail_mcp/tools/update_mail_message_flags_tool_spec.rb
index e6f1419..eaff7aa 100644
--- a/spec/mail_mcp/tools/update_mail_message_flags_tool_spec.rb
+++ b/spec/mail_mcp/tools/update_mail_message_flags_tool_spec.rb
@@ -13,17 +13,59 @@
     allow(MailMCP::ImapClient).to receive(:connect).and_yield(imap_client)
   end
 
-  it "adds flags as symbols" do
+  it "adds system flags as symbols (so net/imap renders them with a backslash)" do
     described_class.call(folder: "INBOX", uid: 5, add: ["\\Seen", "\\Flagged"], server_context: context)
     expect(imap_client).to have_received(:update_flags).with(
-      folder: "INBOX", uid: 5, add: %i[\\Seen \\Flagged], remove: []
+      folder: "INBOX", uid: 5, add: %i[Seen Flagged], remove: []
     )
   end
 
-  it "removes flags as symbols" do
+  it "removes system flags as symbols" do
     described_class.call(folder: "INBOX", uid: 5, remove: ["\\Seen"], server_context: context)
     expect(imap_client).to have_received(:update_flags).with(
-      hash_including(remove: [:"\\Seen"])
+      hash_including(remove: [:Seen])
+    )
+  end
+
+  it "passes custom keywords (e.g. colors) as strings, not system-flag symbols" do
+    described_class.call(folder: "INBOX", uid: 5, add: ["YELLOW", "$label1"], server_context: context)
+    expect(imap_client).to have_received(:update_flags).with(
+      hash_including(add: ["YELLOW", "$label1"])
+    )
+  end
+
+  it "treats system flag names without a backslash as system flags" do
+    described_class.call(folder: "INBOX", uid: 5, add: ["Seen"], server_context: context)
+    expect(imap_client).to have_received(:update_flags).with(
+      hash_including(add: [:Seen])
+    )
+  end
+
+  it "matches system flags case-insensitively and normalizes capitalization" do
+    described_class.call(folder: "INBOX", uid: 5, add: ["\\seen", "FLAGGED"], server_context: context)
+    expect(imap_client).to have_received(:update_flags).with(
+      hash_including(add: %i[Seen Flagged])
+    )
+  end
+
+  it "maps friendly color names to $labelN keywords" do
+    described_class.call(folder: "INBOX", uid: 5, add: %w[red blue], server_context: context)
+    expect(imap_client).to have_received(:update_flags).with(
+      hash_including(add: ["$label1", "$label4"])
+    )
+  end
+
+  it "maps color names case-insensitively, including on remove" do
+    described_class.call(folder: "INBOX", uid: 5, remove: ["GREEN"], server_context: context)
+    expect(imap_client).to have_received(:update_flags).with(
+      hash_including(remove: ["$label3"])
+    )
+  end
+
+  it "passes raw $labelN keywords through unchanged" do
+    described_class.call(folder: "INBOX", uid: 5, add: %w[$label1 $label5], server_context: context)
+    expect(imap_client).to have_received(:update_flags).with(
+      hash_including(add: %w[$label1 $label5])
     )
   end