Opacity property not specified properly

In Ruby 2d - Colors#Opacity, it hasn't been mentioned whether the opacity attribute is applicable for the object, or the color type. It would be great if this can be expanded. I've tried with the object, but it throws an error.

[WillianChuLive]

if the opacity attribute doesn't work as
square.opacity= 0.8

try this way
square = Square.new(x: 10, y: 20, size: 25, color: 'blue', opacity: 0.5)