diff --git a/homeassistant/components/mqtt/light/schema_json.py b/homeassistant/components/mqtt/light/schema_json.py
index 3479f1611d8..b1e5c1c18d4 100644
--- a/homeassistant/components/mqtt/light/schema_json.py
+++ b/homeassistant/components/mqtt/light/schema_json.py
@@ -220,6 +220,8 @@ class MqttLightJson(MqttEntity, LightEntity, RestoreEntity):
             self._attr_supported_color_modes = self._config[CONF_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))
+            else:
+                self._attr_color_mode = ColorMode.UNKNOWN
 
     def _update_color(self, values: dict[str, Any]) -> None:
         if not self._config[CONF_COLOR_MODE]:
diff --git a/tests/components/mqtt/test_light_json.py b/tests/components/mqtt/test_light_json.py
index c5c24c3ae79..d1fa2b72a31 100644
--- a/tests/components/mqtt/test_light_json.py
+++ b/tests/components/mqtt/test_light_json.py
@@ -303,6 +303,80 @@ async def test_single_color_mode(
     assert state.attributes.get(light.ATTR_COLOR_MODE) == color_modes[0]
 
 
+@pytest.mark.parametrize("hass_config", [COLOR_MODES_CONFIG])
+async def test_turn_on_with_unknown_color_mode_optimistic(
+    hass: HomeAssistant,
+    mqtt_mock_entry: MqttMockHAClientGenerator,
+) -> None:
+    """Test setup and turn with unknown color_mode in optimistic mode."""
+    await mqtt_mock_entry()
+    state = hass.states.get("light.test")
+    assert state.state == STATE_UNKNOWN
+
+    # Turn on the light without brightness or color_temp attributes
+    await common.async_turn_on(hass, "light.test")
+    state = hass.states.get("light.test")
+    assert state.attributes.get("color_mode") == light.ColorMode.UNKNOWN
+    assert state.attributes.get("brightness") is None
+    assert state.attributes.get("color_temp") is None
+    assert state.state == STATE_ON
+
+    # Turn on the light with brightness or color_temp attributes
+    await common.async_turn_on(hass, "light.test", brightness=50, color_temp=192)
+    state = hass.states.get("light.test")
+    assert state.attributes.get("color_mode") == light.ColorMode.COLOR_TEMP
+    assert state.attributes.get("brightness") == 50
+    assert state.attributes.get("color_temp") == 192
+    assert state.state == STATE_ON
+
+
+@pytest.mark.parametrize(
+    "hass_config",
+    [
+        (
+            help_custom_config(
+                light.DOMAIN,
+                COLOR_MODES_CONFIG,
+                ({"state_topic": "test_light"},),
+            )
+        )
+    ],
+)
+async def test_controlling_state_with_unknown_color_mode(
+    hass: HomeAssistant,
+    mqtt_mock_entry: MqttMockHAClientGenerator,
+) -> None:
+    """Test setup and turn with unknown color_mode in optimistic mode."""
+    await mqtt_mock_entry()
+    state = hass.states.get("light.test")
+    assert state.state == STATE_UNKNOWN
+
+    # Send `on` state but omit other attributes
+    async_fire_mqtt_message(
+        hass,
+        "test_light",
+        '{"state": "ON"}',
+    )
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+    assert state.attributes.get(light.ATTR_COLOR_TEMP) is None
+    assert state.attributes.get(light.ATTR_BRIGHTNESS) is None
+    assert state.attributes.get(light.ATTR_COLOR_MODE) == light.ColorMode.UNKNOWN
+
+    # Send complete light state
+    async_fire_mqtt_message(
+        hass,
+        "test_light",
+        '{"state": "ON", "brightness": 50, "color_mode": "color_temp", "color_temp": 192}',
+    )
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+
+    assert state.attributes.get(light.ATTR_COLOR_TEMP) == 192
+    assert state.attributes.get(light.ATTR_BRIGHTNESS) == 50
+    assert state.attributes.get(light.ATTR_COLOR_MODE) == light.ColorMode.COLOR_TEMP
+
+
 @pytest.mark.parametrize(
     "hass_config",
     [