A colour or colours that are transparent versions of colour.
Arguments
colour
The R colour (or colours) to be made transparent. May be
specified in any way recognised by col2rgb: a
colour name, a hexadecimal string such as "#ffbc48" or a positive
integer i meaning meaning palette()[i].
transparency
Transparency, from 0, meaning fully opaque, through to
1, which is completely transparent (i.e. invisible).
relative
Determines what happens if colour is already
transparent. If relative is FALSE (the default), then the
transparency value of colour is ignored and transparency defines the
transparency of the returned colour. If TRUE, the existing
transparency value is multiplied by transparency.