You are right. The digit "1" was painted on many machines belonged to pilots of Staffelkapitän rank very often. However it seems it wasn't a rule. Here a few examples ....
Bf 109E-4 'Yellow l' flown by Oberleutnant Gerhard Schopfel, Staffelkapitan of 9./JG 26 'Schlageter', August 1940
Bf 109E-4 'White 1' flown by Oberleutnant Werner Machold, Staffelkapitan 7./JG 2 'Richthofen', September 1940
Bf 109E 'Yellow l' flown by Oberleutnant Josef Priller, Staffelkapitan 6./JG 51, October 1940
Bf 109E 'Black 1' flown by Hauptmann Horst Tietzen, Staffelkapitan 5./JG 51, August 1940
Bf109E-4 'White1' flown by Oberleutnant Hans Philipp, Staffelkapitan 4./JG 54, October 1940
Bf 109E 'Yellow 1' flown by Oberleutnant Wilhelm Moritz, Staffelkapitan 6./JG 77, September 1940
...and a few more earlier and later. Profiles of these Bf109E mentioned above and other examples you can find here...
Aircraft illustration
Why "White 1" for example.....? Please make a focus on the profile of the Bf 109E 'White 13' flown by Feldwebel Heinz Bar,1./JG 51, September 1940 showed at the third page there ( the second from the bottom of the page ). In the Kagero book about Bf109 I have found an image of two Emils camouflaged under trees during the Summer of 1940. Believing in the caption these Bf109Es were of I./JG51 and were marked with "12" and "5". All numbers seem to be of white without the black outline like Bar's "White 13". Also, there is a picture of a cockpit area of a Bf109E of 1./JG51 marked with number "8". The caption says it was "Yellow 8". I think it can be a mistake because yellow numbers had a black outline very often. There isn't a such one in the pic. Moreover... according to my knowledge these white numbers were used by the 1st, 4th, 7th and 10th Staffel that were the first Flights of every Gruppe belonged to a Jagdgeschwader.
The 2nd, 5th, 8th and 11th were marked with black or red numbers. And the 3rd, 6th, 9th and 12th wore yellow ones. So, in my opinion the yellow number "8" couldn't be painted on a machine from 1./JG51