Yellowing white paint

You could try parking it in plain old sunlight for a while and see if the natural light bleaches out the white. However, sunlight isn't picky and tends to bleach out all colors, so don't leave it too long.

Sadly not much can be done to prevent this, other then keeping the kits it a darker room.

Most of the paints we use in this hobby do not include a UV protector to prevent yellowing or fading. Is it the paint yellowing or are their environmental factors contributing to a build up of the yellow tinge to the surface such as nicotine from cigarette smoke. Models attract it like fingers do superglue.

If it is the latter, wash with a mild detergent. Once clean seal with a sealer that includes a UV protector such as Future which has some.

Many of my older models show some yellowing even though they are stored in a protected way.  Whites & light Greys show the yellowing the most.  I notice it especially on models that I used Testors gloss & flat coat lacquers as the final top coats.  Models that I used Future & PollyScale Flat as top coats show significantly less yellowing.

Regards,  Rick

Yellowing (unless caused by soiling as Gerald mentioned) is caused by changes in the binder, not the pigment, which is very stable. It is caused primarily by exposure to ultraviolet radiation, including light from fluorescent bulbs. Clear coats are particularly vulnerable, although some (like Future) are more resistant. However, even the most resistant will yellow given sufficient time or exposure. As Gerald also mentioned, hobby paints rarely contain any anti-yellowing components, nor are their binders generally formulated to resist the effects of ultraviolet.