Rails colored prompt without gems

application.rb

# config/application.rb
module RailsApplicationName
  class Application < Rails::Application
    ...
    console do
      ARGV.push "-r", root.join("lib/console.rb")
    end
  end
end

console.rb

# lib/console.rb

app = "App name"
env = Rails.env
env_color = Rails.env.production? ? "\e[31m#{env}\e[0m" : env # Red (31) if production environment

IRB.conf[:PROMPT] ||= {}
IRB.conf[:PROMPT][:RAILS_APP] = {
  PROMPT_I: "#{app} (#{env_color}) > ",
  PROMPT_N: nil,
  PROMPT_S: nil,
  PROMPT_C: nil,
  RETURN:   "=> %s\n"
}

# Use the custom  prompt
IRB.conf[:PROMPT_MODE] = :RAILS_APP

unless Rails.env.production?
  # Save commands in `~/.app-env-irb-history`
  IRB.conf[:HISTORY_FILE] = File.expand_path("~/.#{app}-#{env}-irb-history")

  # Save 2000 lines of command history
  IRB.conf[:SAVE_HISTORY] = 2000
end