Rainbow is used without having the gem in the Gemfiles:

$ bundle exec rake templates:render DIRECTORY=../community-templates
[...]
rake aborted!
NoMethodError: undefined method `Rainbow' for main:Object
/home/mmoll/mystuff/gitrepos/theforeman/foreman/lib/tasks/templates_renderer.rake:34:in `block (3 levels) in <top (required)>'
/home/mmoll/mystuff/gitrepos/theforeman/foreman/app/models/concerns/foreman/thread_session.rb:108:in `as'
/home/mmoll/mystuff/gitrepos/theforeman/foreman/app/models/concerns/foreman/thread_session.rb:114:in `as_anonymous_admin'
/home/mmoll/mystuff/gitrepos/theforeman/foreman/lib/tasks/templates_renderer.rake:19:in `block (2 levels) in <top (required)>'
/home/mmoll/mystuff/gitrepos/theforeman/foreman/vendor/ruby/2.5.0/gems/rake-13.0.0/exe/rake:27:in `<top (required)>'
Tasks: TOP => templates:render
(See full trace by running task with --trace)