I'm German as well, and I also feel that only Stickies are commonly yellow. But Stickies.app on OS X 10.11 has retained its yellow icon, and it actually makes it easier to tell Notes.app and Stickies.app apart now. (Yes, Stickies.app still has its fans :P)
I think the bigger issue with the new Notes icon is the weak branding. Previously, you could tell Notes and Reminders apart by just looking at the colours. This was really important when you told Siri to "remind me to buy milk tomorrow" - you would either see a bright-yellow note[1], or a black-red-white reminder[2]. Now, everything is "almost white", making it really hard to tell what content lives in which app. (Notes and Reminders have a lot of conceptual overlap, especially now that Notes supports checklists.)
I think the bigger issue with the new Notes icon is the weak branding. Previously, you could tell Notes and Reminders apart by just looking at the colours. This was really important when you told Siri to "remind me to buy milk tomorrow" - you would either see a bright-yellow note[1], or a black-red-white reminder[2]. Now, everything is "almost white", making it really hard to tell what content lives in which app. (Notes and Reminders have a lot of conceptual overlap, especially now that Notes supports checklists.)
[1] http://core0.staticworld.net/images/article/2013/03/siri-not... [2] http://cdn.osxdaily.com/wp-content/uploads/2012/10/location-...