Sorry for asking such a stupid question but I just wanted to make sure.

Should I add yellow or black?
Cheers & thanks in advance,
WD.

I would add small amounts of light grey or linen until the correct hue is attained.

Yellow and/or grey depending on what shade of, off white you want.

If you want to use it for the interior of German WW2 vehicles I suggest adding a drop of Dessert yellow or mid stone through your White. The German Vehicles were painted in the so called Elfenbeincolor which is a bit like Ivory.. White with a little yellow.
Otherwise you can add some grey but always small amounts.

You haven't stated if it's for vehicles or figures..

As the first has already been answered, I'll respond to the second... For white uniforms, I have discovered that a touch of light grey/blue works wonders. Black, IMHO,is just too harsh...Jim

I agree with the above two. It really depends on the subject, and the natural surroundings. Is it for a dio, vehical, or figures? I would/could suggest other ideas if I had more info. PM me if you are interested. I am a trained artist and would be willing to share my secrets.

I should have been more specific.
http://www.airliners.net/search/photo.search?regsearch=305&distinct_entry=true
Look at the white on the Flanker. Thats what I want.

If in a dio sitting on a tarmack I would choose a little yellow ocre. If in flight I would suggest a little bit of blue. My reasoning, if on the ground, the white would reflect the colors of its surroundings, the ground, the sun, and so on. In the air it would reflect more blue from the sky. The photo's look shadowed it appears to be more of a cream color when the plane is shadowed. When in the direct sun the areas appear to be more blue in color. In reality I believe it to be more of a white color and would be more of a blue/grey. Bottom line I would choose white with a hint of the blue/grey tint, very subtle.

Wilco & Thanks.

IIRC, the original color is very light blue. The pictures show the difficulty of getting an exact color match. In one, it looks pure white, in the next a creamy white and in another very light blue.
I'd definitely not use black. Pring out one of the pictures. You would probably be safe mixing a touch of the lightest blue you have to basic white and mix to get the tone you feel is closest. If anyone questions your choice, you have the picture to support your choice.