1.rb

class DefaultAvatarController < ApplicationController
  respond_to :svg

  COLORS = %w( #ff658c #fa80db #d185ee #b385f5 #7075ef #5098ea #a8b6cc #03bee9 #00bdb3
               #00d590 #a0d930 #f3cb00 #efc47b #ff9740 #d8965b #ff755a #3e464e )

  def avatar
    model = params[:model_name].capitalize.constantize.find(params[:id])
    @abbr = model.name.match(/(?:\s*(\w).*?\s+(\w).*)|(?:^\s*(\w)(\w).*)|(\w)/).to_a[1..-1].join('').upcase
    @color = COLORS[model.id % COLORS.size]

    expires_in 1.year, public: true
  end
end

2.rb

class DefaultAvatarController < ApplicationController
  respond_to :svg

  COLORS = %w( #ff658c #fa80db #d185ee #b385f5 #7075ef #5098ea #a8b6cc #03bee9 #00bdb3
               #00d590 #a0d930 #f3cb00 #efc47b #ff9740 #d8965b #ff755a #3e464e )

  NAMED_REGEXP = /(?:\s*(\w).*?\s+(\w).*)|(?:^\s*(\w)(\w).*)|(\w)/

  helper_method :abbr, :color

  def avatar
    expires_in 1.year, public: true
  end

  private

  def model
    @model ||= params[:model_name].capitalize.constantize.find(params[:id])
  end

  def color
    COLORS[model.id % COLORS.size]
  end

  def abbr
    model.name.match(NAMED_REGEXP).to_a[1..-1].join('').upcase
  end
end