White paint gone yellow?

A trick worth trying is to "bleach" the white with a light source strong in ultraviolet. Try placing the model belly-up in a window or under a fluorescent lamp for 2 - 3 days and see if the white loses its yellowish tinge.

Maybe there was a touch of non-slip gelb in it?

I am a lifelong modeller & professional coatings chemist. Paint can turn yellow for various reasons. Chief among these is the selection of base resin used by the paint manufacturer. Older alkyd resins can turn yellow after some time and even some new paint can yellow due to antioxidants not being added to the paint when it is being manufactured. The pigment used to make white paint is called Titanium Dioxide. Low grade Titanium Dioxide (anatase type) can darken over time and give the paint film a yellow tint.

I've had probs years ago with old humbrol white going 'off' & ditto humbrol varnish.

Some painters suggest putting the smallest trace of blue in the paint, to 'blue' it un-noticeably, this stops the yellowing. 

For Varnish, I've been using Future, with no noticeable effects, except when I've wanted the canopy gold, green, or blue tinted.

But have been working with Vallejo paints for the last few years & haven't used the white in any quantity, so can't comment about how well Vallejo white holds up.

I have had Testors white go yellow in the past (more than twenty years ago).  I have not had this problem with the paints I have used in the past couple of decades, but I know of several folks who have noticed this effect with the older paints.  I don't think you will have problems with the paints you get off the hobby shop shelves today.