Creates and returns an NSColor object that has the same color space and component values as the receiver, but the specified alpha component.
Original signature is '- (NSColor *)colorWithAlphaComponent:(CGFloat)alpha'
Available in Mac OS X v10.0 and later.
- alpha (CGFloat)
- The opacity value of the new NSColor object.
- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)