Tarique,
Most of the yellowing we see in old paintings is the varnish, not in the paint, unless the artist has added resins to the paint, or oils boiled with driers. The yellowing of linseed oil bleaches out in the light, and disappears unless the painting is kept in the dark.
Oil paint is still the ultimate artist's medium, despite the eventual embrittlement, as it gives the artist the greatest range of light and dark, color saturation, transparency and opacity choices, plus sufficient working time to refine the images to the highest degree if desired. Acrylics do not allow as high a pigment load, dry too fast, and dry to a different value when they dry than they are when wet. I consider acrylics a commercial art medium, where the fast drying allows the picture to be shipped minutes after the last stroke has been applied.
You will find that there is a lot of incorrect information being passed around as fact, including in books, and it can be very misleading and very confusing. The most reliable book currently in print on artists' materials is
The Painter's Handbook by Mark D. Gottsegen. I am writing one myself, at the urging of many people, but I cannot say when it will be published, as I put painting first. I recommend you buy Mark's book, and refer to it when you have doubts or questions about art materials. He is the Chairman of the ASTM Subcommittee on Artists' Materials, of which I am also a member.
Administrator's Note:
The Painter's Handbook can be purchased online from Amazon.com by clicking on this title.