diff --git a/packages/models-library/src/models_library/projects_nodes_ui.py b/packages/models-library/src/models_library/projects_nodes_ui.py
index e14f2b21a28..d7d284e3afb 100644
--- a/packages/models-library/src/models_library/projects_nodes_ui.py
+++ b/packages/models-library/src/models_library/projects_nodes_ui.py
@@ -2,7 +2,9 @@
     Models node UI (legacy model, use instead projects.ui.py)
 """
 
-from pydantic import BaseModel, ConfigDict, Field
+from typing import Annotated
+
+from pydantic import BaseModel, ConfigDict, Field, PlainSerializer
 from pydantic_extra_types.color import Color
 
 
@@ -14,6 +16,6 @@ class Position(BaseModel):
 
 
 class Marker(BaseModel):
-    color: Color = Field(...)
+    color: Annotated[Color, PlainSerializer(str), Field(...)]
 
     model_config = ConfigDict(extra="forbid")
diff --git a/packages/models-library/src/models_library/projects_ui.py b/packages/models-library/src/models_library/projects_ui.py
index 64556b8c044..a7f6dbc7531 100644
--- a/packages/models-library/src/models_library/projects_ui.py
+++ b/packages/models-library/src/models_library/projects_ui.py
@@ -2,9 +2,9 @@
     Models Front-end UI
 """
 
-from typing import Literal
+from typing import Annotated, Literal
 
-from pydantic import BaseModel, ConfigDict, Field, field_validator
+from pydantic import BaseModel, ConfigDict, Field, PlainSerializer, field_validator
 from pydantic_extra_types.color import Color
 from typing_extensions import (  # https://docs.pydantic.dev/latest/api/standard_library_types/#typeddict
     TypedDict,
@@ -31,7 +31,7 @@ class Slideshow(_SlideshowRequired, total=False):
 
 class Annotation(BaseModel):
     type: Literal["note", "rect", "text"] = Field(...)
-    color: Color = Field(...)
+    color: Annotated[Color, PlainSerializer(str), Field(...)]
     attributes: dict = Field(..., description="svg attributes")
     model_config = ConfigDict(
         extra="forbid",
diff --git a/packages/models-library/tests/test_projects_nodes_ui.py b/packages/models-library/tests/test_projects_nodes_ui.py
new file mode 100644
index 00000000000..3ec33ca677d
--- /dev/null
+++ b/packages/models-library/tests/test_projects_nodes_ui.py
@@ -0,0 +1,8 @@
+from models_library.projects_nodes_ui import Marker
+from pydantic_extra_types.color import Color
+
+
+def test_marker_serialization():
+    m = Marker(color=Color("#b7e28d"))
+
+    assert m.model_dump_json() == '{"color":"#b7e28d"}'