How to debug assets compilation locally

I've got my css mostly written in sass, and there is a lot of it. Using a 3rd party vendor css too.

Turns out when I want to compress it, I get some errors, how does one debug those usually ?

When I execute RAILS_ENV=production rake assets:precompile --trace this happens :

** Invoke assets:precompile (first_time)
** Execute assets:precompile
** Invoke assets:precompile:all (first_time)
** Invoke assets:cache:clean (first_time)
** Invoke assets:environment (first_time)
** Execute assets:environment
** Execute assets:cache:clean
** Execute assets:precompile:all
rake aborted!
Sass::SyntaxError: Invalid CSS after ".": expected class name, was "#ff0"
  (in /Users/gandalf/workspace/zazzy/app/assets/stylesheets/application-print.sass)
/Users/gandalf/.rvm/gems/jruby-1.7.19/bundler/gems/zazul-66fec84bb284/vendor/assets/stylesheets/utilities/_colors.sass:95:in `color'
/Users/gandalf/.rvm/gems/jruby-1.7.19/bundler/gems/zazul-66fec84bb284/vendor/assets/stylesheets/utilities/_colors.sass:101
/Users/gandalf/.rvm/gems/jruby-1.7.19/bundler/gems/zazul-66fec84bb284/vendor/assets/stylesheets/zazul.css.sass:28
/Users/gandalf/workspace/zazzy/app/assets/stylesheets/application-print.sass:5

How does one debug this? And how do I know once I resolved it, can I compile it on it's own? All this css works fine on development by the way, the issues is when I try to compress it. When I add config.assets.compress = true to my configuration. Any hints?