Fix missing color_mode initialization in MQTT JSON light schema

Initialize `_fixed_color_mode` and `_attr_color_mode` before the
color mode branching logic in `_setup_from_config`, matching the
pattern already used in `schema_template.py` (added in #162715).

Without this, `_attr_color_mode` can remain `None` (the new default
from #162715) in edge cases during entity (re)setup, causing
"does not report a color mode" errors when the light is on.

Also use `_fixed_color_mode` in `_update_color()` to skip color
mode changes for single-mode lights, preventing the fixed mode
from being overwritten by state updates. This matches the guard
pattern used in `schema_template.py`.

Author: noerstad

homeassistant/components/mqtt/light/schema_json.py

@@ -190,10 +190,13 @@ def _setup_from_config(self, config: ConfigType) -> None:
         self._attr_supported_features |= (
             config[CONF_TRANSITION] and LightEntityFeature.TRANSITION
         )
+        self._fixed_color_mode = None
+        self._attr_color_mode = ColorMode.UNKNOWN
         if supported_color_modes := self._config.get(CONF_SUPPORTED_COLOR_MODES):
             self._attr_supported_color_modes = supported_color_modes
             if self.supported_color_modes and len(self.supported_color_modes) == 1:
-                self._attr_color_mode = next(iter(self.supported_color_modes))
+                self._fixed_color_mode = next(iter(self.supported_color_modes))
+                self._attr_color_mode = self._fixed_color_mode
             else:
                 self._attr_color_mode = ColorMode.UNKNOWN
         elif config.get(CONF_BRIGHTNESS):
@@ -206,7 +209,7 @@ def _setup_from_config(self, config: ConfigType) -> None:
             self._attr_color_mode = ColorMode.ONOFF
 
     def _update_color(self, values: dict[str, Any]) -> None:
-        color_mode: str = values["color_mode"]
+        color_mode: str = self._fixed_color_mode or values["color_mode"]
         if not self._supports_color_mode(color_mode):
             _LOGGER.warning(
                 "Invalid color mode '%s' received for entity %s",
@@ -337,8 +340,12 @@ async def _subscribe_topics(self) -> None:
             self._attr_brightness = last_attributes.get(
                 ATTR_BRIGHTNESS, self.brightness
             )
-            self._attr_color_mode = last_attributes.get(
-                ATTR_COLOR_MODE, self.color_mode
+            self._attr_color_mode = cast(
+                ColorMode,
+                self._fixed_color_mode
+                if self._fixed_color_mode is not None
+                else last_attributes.get(ATTR_COLOR_MODE, self.color_mode)
+                or ColorMode.UNKNOWN,
             )
             self._attr_color_temp_kelvin = last_attributes.get(
                 ATTR_COLOR_TEMP_KELVIN, self.color_temp_kelvin

tests/components/mqtt/test_light_json.py

@@ -513,6 +513,61 @@ async def test_brightness_only(
     assert state.state == STATE_OFF
 
 
+@pytest.mark.parametrize(
+    "hass_config",
+    [
+        {
+            mqtt.DOMAIN: {
+                light.DOMAIN: {
+                    "schema": "json",
+                    "name": "test",
+                    "state_topic": "test_light_rgb",
+                    "command_topic": "test_light_rgb/set",
+                    "supported_color_modes": ["brightness"],
+                }
+            }
+        },
+        {
+            mqtt.DOMAIN: {
+                light.DOMAIN: {
+                    "schema": "json",
+                    "name": "test",
+                    "state_topic": "test_light_rgb",
+                    "command_topic": "test_light_rgb/set",
+                    "supported_color_modes": ["color_temp"],
+                }
+            }
+        },
+    ],
+)
+async def test_single_color_mode_turn_on(
+    hass: HomeAssistant, mqtt_mock_entry: MqttMockHAClientGenerator
+) -> None:
+    """Test turning on a single color mode light does not raise.
+
+    Regression test: PR #162715 changed _attr_color_mode default to None
+    and added a strict check. The JSON schema must initialize color_mode
+    during setup so that turn_on does not raise "does not report a color mode".
+    """
+    await mqtt_mock_entry()
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_UNKNOWN
+
+    async_fire_mqtt_message(hass, "test_light_rgb", '{"state":"ON", "brightness": 50}')
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+
+    # This should not raise "does not report a color mode"
+    await common.async_turn_on(hass, "light.test")
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+    assert state.attributes.get(light.ATTR_COLOR_MODE) is not None
+    assert state.attributes.get(light.ATTR_COLOR_MODE) != light.ColorMode.UNKNOWN
+
+
 @pytest.mark.parametrize(
     "hass_config",
     [