Intro
I lived here for about 2.5 years as both a student in college and working expat. It's one of the hidden gems in Germany, and tourists are usually hard to come by, making foreigner status a huge advantage. The Cologne/Bonn area is more or less one city, with students and professionals traveling back and forth on a daily basis. These two cities make up the "fun" area of Nordrhein-Westfalen, which is otherwise very industrial and grungy (Düsseldorf, Essen, etc.).
I will update the thread later with information on Cologne venues, more to come.
Culture
Germans here are neither friendly nor cold but somewhere in the middle. If you've spent time in Munich, you'll find most people a bit dull and unexcitable. On the other hand, it's very open and social compared to northern cities like Bremen and Hamburg.
Traditions are important here. The annual "Karneval" in February is an absolute fuckfest, something I can only compare to Mardi Gras in the U.S. but with less of a social stigma on indescriminate stranger banging. The streets are filled with people in Halloween-style clown costumes including a large student population, and partying goes on for several days.
The language here tends to be of a softer or less harsh dialect compared to the quickness of Berlin or the rhythm of Bayrisch. If you're new to the language, this should be a great starting point as most of younger crowd speaks standard Hochdeutsch. A great opener here is walking up to a girl and asking for a lighter, but in a very low-class Turkish accent ("Hasfeuerbrüda?" instead of "Hast du Feuer?") It's a very politically correct society, and little cracks like this go a long way in getting Germans to loosen up.
Demographics
You can find 1-2 million people in the area, spread throughout Cologne and the neighboring cities (Bonn, Leverkusen, etc.) As it's an industrial area bombed to hell during WW2, there's a very large Turkish / North African population. Even though crime is pretty low, it's not regarded as one of the most "German" areas of Germany anymore and has the industrial feel of Cleveland or Detroit.
Fortunately, these are massive student cities and there are more 18-25 year old girls than you can possibly count. Quantity is never a problem, ratios are generally solid, but quality can be a bit of an issue. High on average, but I'm very often surprised by the lack of stunners. Mostly brunettes; blondes are rare compared to Hamburg or the east of the country.
Female Bitch Scale
It's difficult to describe the girls here because the dating culture skews the frame of reference. Compared to other cities in Germany, girls are friendly and open to conversation with strangers, but have massive bitch shields when it comes to first-night bangs. Club sluts and other exceptions obviously abound, but ONS's are harder to come by with German girls unless you're after ERASMUS exchange students. Think of German girls as coconuts - thick shells on the outside, very hard to get the first crack, but when you break the shell, everything comes out and you have her locked down for all the time in the world.
Our black RVF members will have no problem in the Cologne area, but I've seen Asian friends of mine have some very fucking long winters. White guys with dark hair do well. Hispanic or Middle Eastern / Persian guys (for lack of a better term, "brown") could be a disadvantage unless you immediately separate yourself from the immigrant population through clothing and hair. Like anywhere in Germany, regardless what physique you have, you're going to struggle unless you dress well.
Logistics
Trains run regularly between Cologne and Bonn (every 20 minutes), and both cities are situated within 30 minutes from major airports (Düsseldorf, Köln/Bonn). The cities themselves have fantastic infrastructure, and S-Bahn and U-Bahn lines connect everything. The bus system in Bonn is also great, especially at night or on weekends.
Bonn is very small and while there aren't many venues, logistics make it one of my favorite cities in Germany. After a night out, you can go home to most parts of the city in as few as 10-15 minutes. Cologne is a larger, more spread out city and the transportation is noticeably worse.
Note: Ratios here have a tendency to fluctuate in a short time. Germans stick to walled-off social circles so large groups will disappear or reappear all at once.
Venues - Bonn
Nachtschicht - Mostly party/electro/hip hop and a younger crowd of students. This is one of the trashier places, but lays are easy to come by, especially from foreign exchange students. Turks will usually flood the place towards the end of the night looking for drunk girls and starting fights, so the golden window is between 1-3 AM.
Carpe Diem - Same music as Nachtschicht, but cleaner and with a slightly less rowdy/trashy crowd. Another easy venue for student bangs but most girls tend to arrive in huge groups (compared to American girls, this is a cockblock death sentence)
3 Raum Wohnung - Smaller venue with more neutral party music, fantastic lounge area. I always recommend going here after meeting girls in other clubs/bars as it's easy to escalate on the couches and sofas. Fair mixture between students and native Germans.
Tante Rike / Kreuzberg - Two clubs with very similar techno/dance music and live DJ's. Lots of drugged up students, but otherwise, very clean places. Most of the girls here tend to be on the older end of the student spectrum (22-25) and most are native Germans.
Day Game
In Cologne, there are fields near the university (just get off at the "Universität" S-Bahn stop) where students crash and relax. These fields and the neighboring streets are good for day game on students, but you'll do just as well in the cafes or shopping centers. There really isn't a bad place to day game in Germany, because girls are all equally awkward and unprepared when a guy approaches them with interest (deer in headlights effect; German guys stick to clubbing). There's very little room for error with German girls and humor often goes over their heads, best to stick to logistical questions and expand as she picks up on your accent.
In Bonn, again, pretty much anywhere. The Hofgarten, a big field in front of the university building, is solid in the summer while the smaller cafes and bars have good numbers.