Welcome to the Forum!
Pix is right on. He is the resident Luftwaffe Jedi Master, and he has posted many fine Bf 109's on this site.
I totally agree with Pix about laying down a white base coat.
I prefer Floquil's Reefer (flat) White, which sprays very well right from the bottle.
For the yellow, I use Aircraft Colors RLM 04 (the old Aeromaster formula). You can get it from either www.squadron.com , or www.greatmodels.com .
This paint is excellent, and again can be sprayed straight from the bottle (assuming you have a fairly good quality airbrush).
As Pix said, you must be patient, and lay it down in very fine coats. Don't get in a hurry (as I have done) to get a depth of color too soon. It may take three or four light coats to achieve the look you want.
Here's a pic of my Dornier using the above described paints:
All the yellow on this model was done as described above.
Hope this helps.
stinger