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Thanks for the feedback! I understand that the diff is quite large and thus harder to review. After attempting to resolve the conflicts between colors, cmap, and multiclass_colors, the logic became complicated. As a result, I had to rewrite the original structure, which unfortunately made the diff harder to follow.

And to clarify: matplotlib hasn't deprecated either cmap or colors — both are still valid and serve different purposes. That said, I agree it's better to recommend cmap as the preferred option, especially for continuous or gradient-based plots.

Below is a breakdown of the changes for clarity:

Original Logic

if (
    isinstance(self.multiclass_colors, str)
    or self.multiclass_colors is None
):
    ...
    if cmap == "tab10" and n_responses <= 10:
        colors = ...
    elif cmap == "tab20" and n_responses <= 20:
        colors = ...
    else:
        colors = ...
elif isinstance(self.multiclass_colors, str):
    colors = ...
else:  # self.multiclass_colors is a list
    colors = ...

• The elif isinstance(self.multiclass_colors, str) is redundant, since that condition is already covered by the first if. I removed it to simplify the structure.
• There was no check for the case when users pass in a continuous colormap (LinearSegmentedColormap), which lacks the .colors attribute. Attempting to access colors from such a colormap would raise an error. This code was originally assuming ListedColormap.
• The original implementation didn’t properly handle conflicts when users specify both cmap/colors and multiclass_colors. Since colors and cmap are passed directly to Matplotlib, while multiclass_colors is handled by scikit-learn, some combinations could lead to ambiguity or silent failures.

Updated Logic (New Version):

if self.multiclass_colors is None:
    if "cmap" in kwargs and "colors" in kwargs:
        raise ValueError(...)
    if "cmap" in kwargs:
        ...
    elif "colors" in kwargs:
        ...
    else:
        ...
else:
    if "cmap" in kwargs:
        warnings.warn(...)  # 'cmap' will be ignored
    if "colors" in kwargs:
        warnings.warn(...)  # 'colors' will be ignored
    if isinstance(self.multiclass_colors, str):
        ...
    elif isinstance(self.multiclass_colors, list):
        ...
    else:
        raise ValueError(...)

This new structure separates the logic into two clear branches:

• When multiclass_colors is not set, it handles cmap and colors mutually exclusively and safely.
• When multiclass_colors is set, it explicitly ignores any cmap or colors in kwargs and processes multiclass_colors accordingly.

This design avoids ambiguity, enforces correct usage, and adds necessary warnings and error handling.

Contour-specific Fix

The actual fix related to the contour plotting method is limited to lines 267 to 271, specifically in the following section:

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
    )

Regarding tests – I will add appropriate test cases to ensure the new logic is properly covered and behaves as expected in all scenarios.

Thanks for the reviews and suggestions, @jeremiedbb and @adrinjalali!