scikit-learn · jeremiedbb · Jul 15, 2025 · Jun 16, 2025 · Jun 16, 2025 · Jun 16, 2025
diff --git a/doc/whats_new/upcoming_changes/sklearn.inspection/31553.fix.rst b/doc/whats_new/upcoming_changes/sklearn.inspection/31553.fix.rst
@@ -0,0 +1,7 @@
+- Fix multiple issues in the multiclass setting of :class:`inspection.DecisionBoundaryDisplay`:
+
+  - `contour` plotting now correctly shows the decision boundary.
+  - `cmap` and `colors` are now properly ignored in favor of `multiclass_colors`.
+  - Linear segmented colormaps are now fully supported.
+
+  By :user:`Yunjie Lin <jshn9515>`
diff --git a/sklearn/inspection/_plot/decision_boundary.py b/sklearn/inspection/_plot/decision_boundary.py
@@ -221,17 +221,22 @@ def plot(self, plot_method="contourf", ax=None, xlabel=None, ylabel=None, **kwar
             self.surface_ = plot_func(self.xx0, self.xx1, self.response, **kwargs)
         else:  # self.response.ndim == 3
             n_responses = self.response.shape[-1]
-            if (
-                isinstance(self.multiclass_colors, str)
-                or self.multiclass_colors is None
+            for kwarg in ("cmap", "colors"):
+                if kwarg in kwargs:
+                    warnings.warn(
+                        f"'{kwarg}' is ignored in favor of 'multiclass_colors' "
+                        "in the multiclass case when the response method is "
+                        "'decision_function' or 'predict_proba'."
+                    )
+                    del kwargs[kwarg]
+
+            if self.multiclass_colors is None or isinstance(
+                self.multiclass_colors, str
             ):
-                if isinstance(self.multiclass_colors, str):
-                    cmap = self.multiclass_colors
+                if self.multiclass_colors is None:
+                    cmap = "tab10" if n_responses <= 10 else "gist_rainbow"
                 else:
-                    if n_responses <= 10:
-                        cmap = "tab10"
-                    else:
-                        cmap = "gist_rainbow"
+                    cmap = self.multiclass_colors
 
                 # Special case for the tab10 and tab20 colormaps that encode a
                 # discrete set of colors that are easily distinguishable
@@ -241,40 +246,41 @@ def plot(self, plot_method="contourf", ax=None, xlabel=None, ylabel=None, **kwar
                 elif cmap == "tab20" and n_responses <= 20:
                     colors = plt.get_cmap("tab20", 20).colors[:n_responses]
                 else:
-                    colors = plt.get_cmap(cmap, n_responses).colors
-            elif isinstance(self.multiclass_colors, str):
-                colors = colors = plt.get_cmap(
-                    self.multiclass_colors, n_responses
-                ).colors
-            else:
+                    cmap = plt.get_cmap(cmap, n_responses)
+                    if not hasattr(cmap, "colors"):
+                        # For LinearSegmentedColormap
+                        colors = cmap(np.linspace(0, 1, n_responses))
+                    else:
+                        colors = cmap.colors
+            elif isinstance(self.multiclass_colors, list):
                 colors = [mpl.colors.to_rgba(color) for color in self.multiclass_colors]
+            else:
+                raise ValueError("'multiclass_colors' must be a list or a str.")
 
             self.multiclass_colors_ = colors
-            multiclass_cmaps = [
-                mpl.colors.LinearSegmentedColormap.from_list(
-                    f"colormap_{class_idx}", [(1.0, 1.0, 1.0, 1.0), (r, g, b, 1.0)]
-                )
-                for class_idx, (r, g, b, _) in enumerate(colors)
-            ]
-
-            self.surface_ = []
-            for class_idx, cmap in enumerate(multiclass_cmaps):
-                response = np.ma.array(
-                    self.response[:, :, class_idx],
-                    mask=~(self.response.argmax(axis=2) == class_idx),
+            if plot_method == "contour":
+                # Plot only argmax map for contour
+                class_map = self.response.argmax(axis=2)
+                self.surface_ = plot_func(
+                    self.xx0, self.xx1, class_map, colors=colors, **kwargs
                 )
-                # `cmap` should not be in kwargs
-                safe_kwargs = kwargs.copy()
-                if "cmap" in safe_kwargs:
-                    del safe_kwargs["cmap"]
-                    warnings.warn(
-                        "Plotting max class of multiclass 'decision_function' or "
-                        "'predict_proba', thus 'multiclass_colors' used and "
-                        "'cmap' kwarg ignored."
+            else:
+                multiclass_cmaps = [
+                    mpl.colors.LinearSegmentedColormap.from_list(
+                        f"colormap_{class_idx}", [(1.0, 1.0, 1.0, 1.0), (r, g, b, 1.0)]
+                    )
+                    for class_idx, (r, g, b, _) in enumerate(colors)
+                ]
+
+                self.surface_ = []
+                for class_idx, cmap in enumerate(multiclass_cmaps):
+                    response = np.ma.array(
+                        self.response[:, :, class_idx],
+                        mask=~(self.response.argmax(axis=2) == class_idx),
+                    )
+                    self.surface_.append(
+                        plot_func(self.xx0, self.xx1, response, cmap=cmap, **kwargs)
                     )
-                self.surface_.append(
-                    plot_func(self.xx0, self.xx1, response, cmap=cmap, **safe_kwargs)
-                )
 
         if xlabel is not None or not ax.get_xlabel():
             xlabel = self.xlabel if xlabel is None else xlabel

diff --git a/sklearn/inspection/_plot/tests/test_boundary_decision_display.py b/sklearn/inspection/_plot/tests/test_boundary_decision_display.py
@@ -169,6 +169,10 @@ def test_input_validation_errors(pyplot, kwargs, error_msg, fitted_clf):
 @pytest.mark.parametrize(
     "kwargs, error_msg",
     [
+        (
+            {"multiclass_colors": {"dict": "not_list"}},
+            "'multiclass_colors' must be a list or a str.",
+        ),
         ({"multiclass_colors": "not_cmap"}, "it must be a valid Matplotlib colormap"),
         ({"multiclass_colors": ["red", "green"]}, "it must be of the same length"),
         (
@@ -617,6 +621,7 @@ def test_multiclass_plot_max_class(pyplot, response_method):
     "multiclass_colors",
     [
         "plasma",
+        "Blues",
         ["red", "green", "blue"],
     ],
 )
@@ -642,31 +647,51 @@ def test_multiclass_colors_cmap(pyplot, plot_method, multiclass_colors):
 
     if multiclass_colors == "plasma":
         colors = mpl.pyplot.get_cmap(multiclass_colors, len(clf.classes_)).colors
+    elif multiclass_colors == "Blues":
+        cmap = mpl.pyplot.get_cmap(multiclass_colors, len(clf.classes_))
+        colors = cmap(np.linspace(0, 1, len(clf.classes_)))
     else:
         colors = [mpl.colors.to_rgba(color) for color in multiclass_colors]
 
-    cmaps = [
-        mpl.colors.LinearSegmentedColormap.from_list(
-            f"colormap_{class_idx}", [(1.0, 1.0, 1.0, 1.0), (r, g, b, 1.0)]
-        )
-        for class_idx, (r, g, b, _) in enumerate(colors)
-    ]
-
-    for idx, quad in enumerate(disp.surface_):
-        assert quad.cmap == cmaps[idx]
+    if plot_method != "contour":
+        cmaps = [
+            mpl.colors.LinearSegmentedColormap.from_list(
+                f"colormap_{class_idx}", [(1.0, 1.0, 1.0, 1.0), (r, g, b, 1.0)]
+            )
+            for class_idx, (r, g, b, _) in enumerate(colors)
+        ]
+        for idx, quad in enumerate(disp.surface_):
+            assert quad.cmap == cmaps[idx]
+    else:
+        assert_allclose(disp.surface_.colors, colors)
 
 
-def test_multiclass_plot_max_class_cmap_kwarg(pyplot):
-    """Check `cmap` kwarg ignored when using plotting max multiclass class."""
+def test_cmap_and_colors_logic(pyplot):
+    """Check the handling logic for `cmap` and `colors`."""
     X, y = load_iris_2d_scaled()
     clf = LogisticRegression().fit(X, y)
 
-    msg = (
-        "Plotting max class of multiclass 'decision_function' or 'predict_proba', "
-        "thus 'multiclass_colors' used and 'cmap' kwarg ignored."
-    )
-    with pytest.warns(UserWarning, match=msg):
-        DecisionBoundaryDisplay.from_estimator(clf, X, cmap="viridis")
+    with pytest.warns(
+        UserWarning,
+        match="'cmap' is ignored in favor of 'multiclass_colors'",
+    ):
+        DecisionBoundaryDisplay.from_estimator(
+            clf,
+            X,
+            multiclass_colors="plasma",
+            cmap="Blues",
+        )
+
+    with pytest.warns(
+        UserWarning,
+        match="'colors' is ignored in favor of 'multiclass_colors'",
+    ):
+        DecisionBoundaryDisplay.from_estimator(
+            clf,
+            X,
+            multiclass_colors="plasma",
+            colors="blue",
+        )
 
 
 def test_subclass_named_constructors_return_type_is_subclass(pyplot):