alpha

The alpha of a CSS <color>, from 0 (transparent) to 1 (opaque). color reads only the RGB part, so a painter that honours transparency asks for both.

CSS Color 4 makes alpha part of the colour: transparent is fully transparent (6.3), rgb() and rgba() take it as a fourth number or percentage (4.2, 5.1), and the four- and eight-digit hex forms carry it in their last digits (5.2). Every other colour is opaque. Null when raw is not a colour color reads.