My matplotlib's version is 3.9.3 and the code below works fine for this color change case.

selector = RectangleSelector(
    ax, onselect,
    useblit=True,
    button=[1],  # Respond only to left mouse button
    interactive=True,
    minspanx=5, minspany=5,
    spancoords='pixels',
    props = dict(facecolor='red', edgecolor = 'white', alpha=0.2, fill=True)
)

enter image description here