sub _split_colours {
  my $color = shift;
  [ map { s/[^\d.-]//g; $_ ||= 0; $_ < 0 ? 0 : $_ / 255 }
        (split(/\s*,\s*/, $color, 4), (0) x 3)[0..2] ];
}

sub your_subroutine {
  my($color, ...) = ( _split_colours($_[0]), @_[1 .. $#_] );
  ...