ActiveDecorator使うときっとこんな感じになりそう(未検証)

equipment_color.rb

# app/decorators/character_decorator.rb

# for https://github.com/kyubuns/misterioso/commit/3598eec678cf7368e105141b1cd7b8c259f1af12

module CharacterDecorator
  EQUIPMENT_COLOR_TABLE = {
    1 => 'black',
    2 => 'orange',
    3 => 'forestgreenq',
    4 => 'blue',
    5 => 'red',
  }
  EQUIPMENT_COLOR_DEFAULT = 'gray'
  EQUIPMENT_COLOR_NOCARD = 'black'
 
  def equipment_color
    if equip_card
      return EQUIPMENT_COLOR_TABLE[equip_card.master_card.rarity] || EQUIPMENT_COLOR_DEFAULT
    end
    EQUIPMENT_COLOR_NOCARD
  end
end

$ pry
[1] pry(main)>
=> nil
[2] pry(main)> module CharacterDecorator
[2] pry(main)* EQUIPMENT_COLOR_TABLE = {
[2] pry(main)* 1 => 'black',
[2] pry(main)* 2 => 'orange',
[2] pry(main)* 3 => 'forestgreenq',
[2] pry(main)* 4 => 'blue',
[2] pry(main)* 5 => 'red',
[2] pry(main)* }
[2] pry(main)* EQUIPMENT_COLOR_DEFAULT = 'gray'
[2] pry(main)* EQUIPMENT_COLOR_NOCARD = 'black'
[2] pry(main)* def equipment_color
[2] pry(main)* EQUIPMENT_COLOR_NOCARD
[2] pry(main)* end
[2] pry(main)* end
=> nil
[6] pry(main)> class Character
[6] pry(main)* include CharacterDecorator
[6] pry(main)* end
=> Character
[7] pry(main)> Character.new.equipment_color
=> "black"

エラー出そうな部分削って試してみた