[RavSinghChandan]

color5.parse_color() is documented to return None for an invalid color without raising, but an invalid alpha in device-cmyk() or in a custom color space crashes:

>>> color5.parse_color('device-cmyk(0 0 0 0 / foo)')
TypeError: float() argument must be a string or a real number, not 'NoneType'
>>> color5.parse_color('color(--custom 1 0 0 / foo)')
TypeError: ...

color4._parse_alpha() returns None for an invalid alpha, which color4.parse_color() checks but color5 passed straight to Color(). It is now checked the same way, so these colors give None. Valid alphas and every other color are unchanged.

Added test_color_invalid_alpha_5; all three cases raise TypeError before the fix

[grewn0uille]

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