A yellow filter will do absolutely zilch to change the contrast. Lith film has a 'tonal range' like any other: from clear to black; A yellow filter won't change that, either.

To lower the contrast with lith film try using Soemarko LC-1 (new formula) developer. From Vaughn's post on the LFP forum:

"From The Book of Alternative Photographic Processes...Second Edition, page 100, by Christopher James:

750ml distilled water at 125F
4 g Metol
80 g Sodium sulfite
4 g Hydroquinone
20 g Sodium bisulfite
add distilled cold water to make 1 liter stock solution

For use, dilute 1:5 to 1:10...5 to 10 minutes at 75F"

It works rather well, far better than I would have expected.

You can't buy it, but have to mix it up yourself. Chemicals are available at http://www.artcraftchemicals.com/products/ and http://stores.photoformulary.com/StoreFront.bok