diff --git a/datawrapper/charts/annos.py b/datawrapper/charts/annos.py
index 340e9c1..b01dd97 100644
--- a/datawrapper/charts/annos.py
+++ b/datawrapper/charts/annos.py
@@ -1,6 +1,6 @@
 from typing import Any, Literal
 
-from pydantic import BaseModel, ConfigDict, Field, field_validator
+from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
 
 from .enums import ArrowHead, ConnectorLineType, LineInterpolation, StrokeWidth
 
@@ -321,8 +321,10 @@ class AreaFill(BaseModel):
     #: The line to fill upwards to
     to_column: str = Field(alias="to", description="The line to fill upwards to")
 
-    #: The color of the fill
-    color: str = Field(default="#15607a", description="The color of the fill")
+    #: The color of the fill (hex string or palette index)
+    color: str | int = Field(
+        default=0, description="The color of the fill (hex string or palette index)"
+    )
 
     #: The opacity of the fill
     opacity: float = Field(default=0.3, description="The opacity of the fill")
@@ -334,11 +336,11 @@ class AreaFill(BaseModel):
         description="Whether to use different colors when there are negative values",
     )
 
-    #: The color of the fill when it is negative
-    color_negative: str = Field(
-        default="#cc0000",
+    #: The color of the fill when it is negative (hex string or palette index, None = disabled)
+    color_negative: str | int | None = Field(
+        default=None,
         alias="colorNegative",
-        description="The color of the fill when it is negative",
+        description="The color of the fill when it is negative (hex string or palette index, None = disabled)",
     )
 
     #: The interpolation method to use when drawing lines
@@ -360,21 +362,39 @@ def validate_interpolation(
                 )
         return v
 
+    @model_validator(mode="after")
+    def auto_enable_mixed_colors(self) -> "AreaFill":
+        """Auto-enable use_mixed_colors when color_negative is provided.
+
+        If a user provides a color_negative value (not None),
+        automatically enable use_mixed_colors to make the feature work as expected.
+        """
+        # Only auto-enable if color_negative is provided and use_mixed_colors is False
+        if self.color_negative is not None and not self.use_mixed_colors:
+            self.use_mixed_colors = True
+        return self
+
     def serialize_model(self) -> dict:
         """Serialize the model to a dictionary for the Datawrapper API.
 
         Note: The 'id' field is not included in the output as it's used as the dict key.
+        Only includes colorNegative if it's not None.
         """
-        return {
+        result = {
             "from": self.from_column,
             "to": self.to_column,
             "color": self.color,
             "opacity": self.opacity,
             "useMixedColors": self.use_mixed_colors,
-            "colorNegative": self.color_negative,
             "interpolation": self.interpolation,
         }
 
+        # Only include colorNegative if it's provided (not None)
+        if self.color_negative is not None:
+            result["colorNegative"] = self.color_negative
+
+        return result
+
     @classmethod
     def deserialize_model(cls, api_data: dict[str, dict] | list | None) -> list[dict]:
         """Deserialize area fills from API response format.