You can change the resin from softening (Cation) to Tannin removal (Anion), many companies even mix the two with some success. There are some concerns with tannin removal. General guidelines state that the water should be softened prior to running the water through the Anion resin bed to prevent fouling of the media. Mixed bed systems do not work properly since the Anion migrates on top of the softening bed, so the hard water goes through the Aion first causing potential service issues and media longevity can be a problem. Amine smell(fish odor) is not uncommon with some anion tannin resins, especially at higher pH (>8) Other methods of tannin removal include chlorine injection, contact tank, dechlorinization. This will also add the benefit of acting as a secondary water sanitization method. Try this simple test, take a jar of your yellowish water, add a couple drops of chlorine to it, and see if this breaks down the tannins. Depending on the cause of the tannins, (what type of vegitation etc is causing it), you should see some settling.
Lastly, do you have a recent test? Considering it is a lake, tannins are probably what is causing the discoloration. Does it have a smell?