The docs give examples of various forms of Color.rgb() with 3 arguments, but never give an example of specifying an alpha value in the constructor. They do give the example Color('#7743CE').alpha(0.5) but nothing like Color.rgb([0x77, 0x43, 0xCE, 0.5]).
Also, there are no examples of construction from non-RGB color spaces. Color.hsl(200,80,70) works but is never mentioned.