colour <-fade: factor=[ -> 0.0..1.0] ->
colour Return a colour with the same RGB but alpha multiplied by
factor. 0.0 yields a fully
transparent variant; 1.0 leaves alpha
unchanged. The default factor comes from the
fade_factor class variable
(0.5).
Complements graphical->opacity: setting
the fill or pen to a faded colour lets a single graphical carry both an
alpha component and its shape/stroke identity, without a global
graphical opacity slot. Like colour<-hilite
and colour<-reduce,
the result is cached against the source colour via @colours.
