diff --git a/src/ansi_string/ansi_string.py b/src/ansi_string/ansi_string.py
index 32a728b..094f032 100644
--- a/src/ansi_string/ansi_string.py
+++ b/src/ansi_string/ansi_string.py
@@ -969,21 +969,15 @@ def apply_formatting(
             return
 
         if end is None:
-            length = None
-        else:
-            length = end - start
-            if length <= 0:
-                # Ignore - nothing to apply
-                return
+            end = len(self._s)
 
         settings = __class__.Settings(setting_or_settings)
 
         # Apply settings
         self._insert_settings(start, True, settings, topmost)
 
-        if length is not None:
-            # Remove settings
-            self._insert_settings(start + length, False, settings, topmost)
+        # Remove settings
+        self._insert_settings(end, False, settings, topmost)
 
     def apply_formatting_for_match(
             self,
@@ -1282,8 +1276,12 @@ def __iadd__(self, value):
             settings_cpy = copy.deepcopy(value._color_settings)
             __class__._shift_settings_idx(settings_cpy, len(self._s), False)
             self._s += value._s
-            for key, value in settings_cpy.values():
-                self._color_settings[key] = value
+            for key, value in settings_cpy.items():
+                if key in self._color_settings:
+                    self._color_settings[key][0].extend(value[0])
+                    self._color_settings[key][1].extend(value[1])
+                else:
+                    self._color_settings[key] = value
         else:
             raise ValueError(f'value is invalid type: {type(value)}')
         return self
diff --git a/tests/test_ansi_string.py b/tests/test_ansi_string.py
index b377851..2063445 100755
--- a/tests/test_ansi_string.py
+++ b/tests/test_ansi_string.py
@@ -63,6 +63,13 @@ def test_ranges(self):
             'across different ranges'
         )
 
+    def test_format_right_only(self):
+        s = AnsiString('This has no ANSI formatting')
+        self.assertEqual(
+            f'{s:#>90}',
+            '###############################################################This has no ANSI formatting'
+        )
+
     def test_format_right_justify_and_int(self):
         s = AnsiString('This string will be formatted bold and red, right justify')
         self.assertEqual(
@@ -101,5 +108,21 @@ def test_no_format_and_rgb_functions2(self):
             '\x1b[38;2;138;43;226;48;2;100;232;170;4;58;2;255;99;71mManually adjust colors of foreground, background, and underline\x1b[m'
         )
 
+    def test_add(self):
+        s = AnsiString('bold', 'bold') + AnsiString('red', 'red')
+        self.assertEqual(
+            str(s),
+            '\x1b[1mbold\x1b[0;31mred\x1b[m'
+        )
+
+    def test_iadd(self):
+        s = AnsiString('part bold')
+        s.apply_formatting(AnsiFormat.BOLD, 0, 3)
+        s += AnsiString('red', 'red')
+        self.assertEqual(
+            str(s),
+            '\x1b[1mpar\x1b[mt bold\x1b[31mred\x1b[m'
+        )
+
 if __name__ == '__main__':
     unittest.main()