Germany originally planned to bring Turkish guest workers for two years at a time on a non-immigration basis. Maintaining that temporary status proved impossible. In the oughts, Germany had to change its naturalization laws to provide a mechanism for permanent residency and citizenship. But decades of living as second-class non-citizens resulted in lack of assimilation: http://www.spiegel.de/international/germany/turkish-immigrat....

(The integration part is a touchy subject in Germany. My point more directed at the idea that you can have a country with a permanent class of non-citizen immigrant workers. You probably cannot do that in a liberal Western democracy.)

I lived in Germany for a few months so I got to have a few conversations. Most of their transport network was built by Turkish men that were imported. That's why Turkish street food is everywhere. They also were not able to bring their families as migrant workers but I think lots slowly migrated in the late 80s to early 00s.

They also have had issues with "assimilation" for every ethnic culture but lately MENA has caused a divide. It resulted in them limiting their intake. You can read a lot about it via the recent refugee crisis. There were stories of gangs of migrants roaming the streets, I only heard about a case of rape but there might be more. I don't think its right to throw everyone together in the clump referenced. However, from talking with folks there — their collective German culture and German ways of doing things is very important to them. If you don't act or think German you certainly are an outsider. I also never got the vibe it was ever a skin color thing, but it is a language and cultural assimilation thing.