For anyone who's interested, here's the Jaad/davido pool-solution in Perl:

package Chat::Pool;

# 16 should suffice.
use constant POOL_SIZE => 16;

sub new { bless [
  # $self->pool: Arrayref of ["#fg","#bg"]-pairs.
  # Fixed size.
  [ $_[0]->precalc_colors(POOL_SIZE) ],
  # $self->col->[$id] contains the ["#fg","#bg"]-pair of a user.
  [],
] => shift }

sub tick {
  my ($self, $id, $time) = @_;

  # If we haven't allocated a color for $id...
  unless(defined $self->col->[$id]) {
    # Take one from the pool (pop), assign in to $id, and unshift it.
    # All in one line :)
    unshift @{ $self->pool }, $self->col->[$id] = pop @{ $self->pool }
+;
  }
}

# Return the allocated color for $_[1].
sub color {@{ $_[0]->col->[$_[1]] }}

# Precalculate the pool.
sub precalc_colors {
  my ($self, $num) = @_;

  local $_;
  return
    map {[ $self->calc_color($_, $num) ]} 0..$num-1;
}

# calc_color as in OP

sub pool : lvalue { $_[0]->[0] }
sub col  : lvalue { $_[0]->[1] }
[download]

This solution is really easy :)

Update: If you don't want HTML-colors, but ANSI (e.g. for STDOUT), you can use:

package Chat::Pool::ANSI;
use base "Chat::Pool";

sub precalc_colors {(
  ["\033[31;1m"],
  ["\033[32;1m"],
  ["\033[33;1m"],
  ["\033[34;1m"],
  ["\033[35;1m"],
  ["\033[36;1m"],
  ["\033[37;1m"],
  ["\033[31m"],
  ["\033[32m"],
  ["\033[33m"],
  ["\033[34m"],
  ["\033[35m"],
  ["\033[36m"],
)}

# later...
printf "%s%s%s\n",
  ($chat->color($id))[0],
  $text,
  "\033[0m";
[download]