_form.html.erb

<%= form_for (@products) do |f| %>
<%= f.label "Colors" %>
	<% for color in Color.all %>
	<%= check_box_tag 'product[color_ids][]', color.id,
	@product.color_ids.include?(color.id), :id => dom_id(color) %>
	<%= label_tag dom_id(colore), color.name, :class => "check_box_label" %>
	<br />
<% end %>

color.rb

class Place < ActiveRecord::Base  
  has_and_belongs_to_many :products
end

products.rb

class Contract < ActiveRecord::Base
  has_and_belongs_to_many :colors
end

schema.rb

create_table "products", :force => true do |t|
    t.string   "color"
  end

create_table "products_colors", :id => false, :force => true do |t|
    t.integer "product_id"
    t.integer "color_id"
  end