An, ahem, answer, of sorts for how to change default IRB colors (irc question)

oh_dear.rb

changes = {IRB::Color::BLUE => IRB::Color::YELLOW}

IRB::Color.const_get(:TOKEN_SEQ_EXPRS).tap do |token_seq_exprs|
  changes.each do |old_color, new_color|
    token_seq_exprs.filter_map do |type, ((color, _), _)|
      color == old_color && type
    end.each do |matching_type|
      token_seq_exprs.dig(matching_type, 0)[0] = new_color
    end
  end
end