How to remove the yellowish color from my old drawings?
I don't believe there is a way to remove the yellow staining but there are a lot of methods to keep it from getting any worse.
The reason your paper is turning yellow is because of the quality of paper that was used to make the drawing on. Economical paper is made from wood and not cotton. Chances are you have paper made from a wood pulp. Wood is made up of lignin and cellulose. The yellowing in the result of lignin, still present in the fibres of your paper, oxidizing when exposed to sunlight and air.
And if the paper wasn't washed properly, residual acids from the paper-making process may still be present. These acids help with the oxidation process.
To save your paper I would recommend two things, 1) Neutralize the residual acids and 2) Seal the paper from contact with the air.
Krylon makes a spray (Make It Acid-Free), that helps neutralize the acids. Neutralizing the acids slows down the yellowing and aging of the paper.

Krylon also makes a preservative for protecting documents and photos called PreserveIt.
Neutralize the acids first, then preserve it.
In the future if you want your drawings to last longer (say a couple of hundred years), try using paper that is archival safe or acid-free.

Editor's note: it looks like Make It Acid-Free may no longer be available.  If that's the case, there are a few other similar products, including Archival Mist and Bookkeeper (which is a liquid used in a hand sprayer).  This is provided only as information; no personal experience with the products.