The way professional finishers would adjust color like this is with a glaze or toner. A glaze is color that's added between coats of finish. You apply it, manipulate while still wet, let it dry, then top coat with a coating compatible with the under-coat. If these are factory chairs, it's probably some type of lacquer.

A toner is finish with color in it. To get it on evenly, you almost certainly have to spray it. Read: forget Polyshades. These you can make yourself, or you can buy in aerosol cans. Very light application because they go from not quite there to way past right in a flash.

With either method, you can only move the color slightly before it starts looking opaque. So if your understanding of "slightly" is the same as mine, either might work.

You are not likely to find products or knowledgeable advice at the big box or chain paint stores. I'd recommend getting the library or book store and looking up the books by Bob Flexner or Jeff Jewitt for more thorough technique information. Then practice on scrap or the bottom of the seat.

To neutralize yellow, you use a product with some purple in it.

Here is a link that might be useful: http://www.popularwoodworking.com/features/finish6.html

Thank you very much for the advice and link.