PR summary

This PR fixes issue #22231 where ax.grid(color=(R, G, B, A)) ignores the alpha channel in color tuples, causing gridlines to appear fully opaque instead of with the specified transparency.

Problem: When calling ax.grid(color=(0.8, 0.8, 0.8, 0.2)), the gridlines appeared with alpha=1.0 instead of alpha=0.2.

Root Cause: The issue occurred in two places in lib/matplotlib/axis.py:

1. Tick.__init__: When creating ticks with a color containing alpha, the code didn't extract the alpha value from the color tuple
2. Tick._apply_params: When updating gridlines via grid(), color tuples with alpha weren't being decomposed

Solution: Extract alpha from color tuples using mcolors.to_rgba() and set color (RGB) and alpha separately, ensuring proper alpha precedence: explicit alpha parameter > color tuple alpha > rcParams.

Minimum example demonstrating the fix:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([0, 1], [0, 1])
ax.grid(True, color=(0.8, 0.8, 0.8, 0.2))  # Alpha=0.2 now works!
plt.show()

closes #22231

PR checklist

• "closes [Bug]: Axes.grid(color) ignores alpha #22231" is in the body of the PR description to link the related issue
• new and changed code is tested (added test_grid_color_with_alpha)
• [N/A] Plotting related features are demonstrated in an example (bug fix, not new feature)
• [N/A] New Features and API Changes are noted with a directive and release note (bug fix, not API change)
• Documentation complies with general and docstring guidelines (no docstring changes needed)