hi i justed started modeling and chose a Cobra from MasterCraft.
But do you gusy have any tips on painting with revell paint,i have paint thinner but how will i use it?
I decided to use a navy blue color on the Cobra.
http://www.mastercraft.com.pl/mastercraft/seria2/B-32a.htm

Navy blue? Army Cobras were all some form of olive drab. Marine Cobras might be a greener drab. 2-3 shades of gray, or gray with brown. Israeli Cobras were light brown.
Do you have reference for a blue Cobra? Certainly the two represeted by the kit would have been dark olive drab.

so first 2 layers of grey and then a layer of brown
revell paint right?

Revell makes an OD Green paint. That is the color you should use.

Not sure of the markings on "Playboy"' but pretty sure it didn't have the colors you are refering to. Any other referance pics to look at and help you out with colors?

From the drawing at the link you posted...

It looks like standard overall OD Green with a white Playboy Bunny on the sail, anti-glare flat black panel on the nose and white lettering of "Playboy", a white diamond tactical marking on the tail boom, and standard black stenciling.

Thhise colors would only apply to a Desert Storm Marine AH-1W "Whixkey Cobra," not your G model. As Ginos says above, the G would have been in Dark OD with black lettering.

Hello maik and welcome to Armorama. As to the question of painting your Cobra navy blue, well your not clear on what country your building it for. The Spanish Navy's Cobra's are painted in an almost navy blue color, Fire Ball Model Works make a decal sheet for that country in those colors.

www.fireballmodels.info

As for using thinners, gloss 3 parts paint 2 parts thinners and for flats 3 parts paint 1 part thinner. I usually mix and thin my paints to the consistencies of milk.