diff --git a/optuna/visualization/_contour.py b/optuna/visualization/_contour.py
index 90f1f3f945b..3eedfdf63c3 100644
--- a/optuna/visualization/_contour.py
+++ b/optuna/visualization/_contour.py
@@ -249,7 +249,7 @@ def _get_contour_subplot(
 
 
 def _create_scatter(x: list[Any], y: list[Any], is_feasible: bool) -> Scatter:
-    edge_color = "Gray" if is_feasible else "#cccccc"
+    edge_color = "Gray"
     marker_color = "black" if is_feasible else "#cccccc"
     name = "Feasible Trial" if is_feasible else "Infeasible Trial"
     return go.Scatter(
diff --git a/optuna/visualization/matplotlib/_contour.py b/optuna/visualization/matplotlib/_contour.py
index ae14bbf3a95..b32a32e00fe 100644
--- a/optuna/visualization/matplotlib/_contour.py
+++ b/optuna/visualization/matplotlib/_contour.py
@@ -332,7 +332,7 @@ def _generate_contour_subplot(info: _SubContourInfo, ax: "Axes", cmap: "Colormap
                 marker="o",
                 c="#cccccc",
                 s=20,
-                edgecolors="#cccccc",
+                edgecolors="grey",
                 linewidth=2.0,
             )