I love markings questions.
During the early war period of 1939, about a year prior to the Battle of Britain, the Luftwaffe introduced a new camouflage which was dark: RLM 70 / 71 and the underside RLM 65.
These are the dark green Bf 109s you see during the Invasion of Poland.
These were great defensive markings, as they hid aircraft against the ground. But they weren't great offensive markings. In air to air combat, they needed something lighter, so they wouldn't be as conspicuous. So, during the winter of 1939/1940, they limited the dark part of the camo to just the upper surfaces of the aircraft. This way, the lighter underside would come up along the sides, giving it a "lower visibility profile".
At the same time, the dark camo on the top was given contrasting colors to break up the natural shape of the aircraft. This contrasting pattern was RLM 71, mixed with 02.
It's these markings (or variations of these markings) that the Luftwaffe JGs entered into the Battle of France with (Spring 1940):
At the same time, new ideas were being developed (on both a command and unit level) with regards to recognition markings. While the aircraft needed to be less conspicuous, it also needed to be quickly identified.
So, the national markings (balkenkreuz and hakenkreuz) were widened with a larger white outline. And yellow started being introduced to the aircraft. Obviously, recognition marking practices vary from unit to unit, and sometimes pilot to pilot (which is why you see so many variations), but generally, the yellow swatches could be found on the nose, on the wingtips, on the tailplane-tips and on the spinners. The most common practice though was to paint the under cowling and the rudder.
For example:
In general, these are markings that most Bf 109s entered the Battle of Britain with. And this is where the British got the idea for the mythical and supposedly elite "Yellow Nosed Bastards".
However, during the Battle of Britain, the dark tops and the light sides were found to be too easy to spot over the English Channel. And thus, the Jagdgruppen (independent of the Luftflotte command) started using captured stocks of French paint to deaden the aircraft colors, and mottle the sides of the aircraft. This practice, which again was done on the unit level (so a lot of variation), is the start of what we now know as "Luftwaffe Grey". And because of this, by late August 1940, a new camo theme starts to emerge:
Notice the darkened sides, done hastily on the ground.
Here's another version:
This hasty mottling would soon evolve into the famous "Luftwaffe squiggle" or "tiger stripe" which you can see on other aircraft of the period.
However, you can still see the bright yellow noses on some of the aircraft.
Officially, the Luftwaffe continued to allow highly visible yellow recognition markings on the Bf 109 until the Summer of 1941, when a general order was made to all daylight fighter units to stop the practice.
However, many pilots recognized long before the Luftwaffe command did, that the bright yellow noses were just too easy for the enemy to spot. And with the changing nature of the war, some of those units took it upon themselves to self restrict (or self mute) their own offically sanctioned markings. Thus, as they deadened the sides, they also muted the nose. All well before 1941.
This is why the yellow noses are present on some aircraft, but not on others!
For example, this Bf 109F-2 from I./JG3...
...becomes this aircraft (click link), once Hans von Hahn realizes that he doesn't want to be seen:
http://img4.imageshack.us/img4/7575/barb028.jpg
Hope that helps!
-Klai
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