diff --git a/app/models/account.rb b/app/models/account.rb
index b41fe3e..b826226 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -117,6 +117,8 @@ def accent_label_color
   end
 
   def accent_color_rgb
+    return nil unless accent_color.present?
+
     rgb = accent_color.match(/^#(..)(..)(..)$/).captures.map(&:hex)
     "rgb(#{rgb.join(', ')})"
   end
diff --git a/test/models/account_test.rb b/test/models/account_test.rb
new file mode 100644
index 0000000..f12db30
--- /dev/null
+++ b/test/models/account_test.rb
@@ -0,0 +1,112 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class AccountTest < ActiveSupport::TestCase
+  test 'should accept valid hex color for accent_color' do
+    account = Account.new(
+      email: 'test@example.com',
+      password: 'password123',
+      name: 'Test User',
+      slug: 'test-user',
+      accent_color: '#ff0000'
+    )
+    assert account.valid?
+  end
+
+  test 'should reject accent_color with CSS injection attempt' do
+    account = Account.new(
+      email: 'test2@example.com',
+      password: 'password123',
+      name: 'Test User 2',
+      slug: 'test-user-2',
+      accent_color: 'red; } body { display:none } .x {'
+    )
+    assert_not account.valid?
+    assert account.errors[:accent_color].present?
+  end
+
+  test 'should reject accent_color with closing style tag' do
+    account = Account.new(
+      email: 'test3@example.com',
+      password: 'password123',
+      name: 'Test User 3',
+      slug: 'test-user-3',
+      accent_color: '</style><script>alert("XSS")</script>'
+    )
+    assert_not account.valid?
+    assert account.errors[:accent_color].present?
+  end
+
+  test 'should reject invalid hex colors' do
+    invalid_colors = [
+      'red',           # Named color
+      '#ff00',         # Too short
+      '#ff00000',      # Too long
+      '#gggggg',       # Invalid hex characters
+      'rgb(255,0,0)',  # RGB format
+      '#ff00;',        # Hex with semicolon
+      '000000',        # Missing #
+    ]
+
+    invalid_colors.each do |color|
+      account = Account.new(
+        email: "test-#{color.gsub(/[^a-z0-9]/, '')}@example.com",
+        password: 'password123',
+        name: 'Test User',
+        slug: "test-user-#{color.gsub(/[^a-z0-9]/, '')}",
+        accent_color: color
+      )
+      assert_not account.valid?, "#{color} should be invalid"
+      assert account.errors[:accent_color].present?, "#{color} should have accent_color error"
+    end
+  end
+
+  test 'should accept valid hex colors with lowercase' do
+    account = Account.new(
+      email: 'test4@example.com',
+      password: 'password123',
+      name: 'Test User 4',
+      slug: 'test-user-4',
+      accent_color: '#abc123'
+    )
+    assert account.valid?
+  end
+
+  test 'should accept valid hex colors with uppercase' do
+    account = Account.new(
+      email: 'test5@example.com',
+      password: 'password123',
+      name: 'Test User 5',
+      slug: 'test-user-5',
+      accent_color: '#ABC123'
+    )
+    assert account.valid?
+  end
+
+  test 'should allow nil accent_color and set random color on save' do
+    account = Account.new(
+      email: 'test6@example.com',
+      password: 'password123',
+      name: 'Test User 6',
+      slug: 'test-user-6',
+      accent_color: nil
+    )
+    # The validation allows nil
+    assert account.valid?
+    # But the set_random_accent_color callback should set a color on save
+    # (We don't save here to avoid database interactions in this unit test)
+  end
+
+  test 'accent_color_rgb returns nil when accent_color is nil' do
+    account = Account.new
+    account.accent_color = nil
+    assert_nil account.accent_color_rgb
+  end
+
+  test 'accent_color_rgb converts valid hex to rgb' do
+    account = Account.new
+    account.accent_color = '#ff0000'
+    assert_equal 'rgb(255, 0, 0)', account.accent_color_rgb
+  end
+end
diff --git a/test/test_helper.rb b/test/test_helper.rb
new file mode 100644
index 0000000..b96d6d3
--- /dev/null
+++ b/test/test_helper.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+ENV['RAILS_ENV'] ||= 'test'
+require_relative '../config/environment'
+require 'rails/test_help'
+
+module ActiveSupport
+  class TestCase
+    # Run tests in parallel with specified workers
+    parallelize(workers: :number_of_processors)
+
+    # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
+    fixtures :all if defined?(fixtures)
+
+    # Add more helper methods to be used by all tests here...
+  end
+end