Every year there is this period of off colour beans as the beans switch over. The field should green up in a week or so. Poor nodulation is a possibility. Count the nodules - there should be 10 to 12 per plant. Cut a few nodules open and check their colour.
Active nodules are pink to an almost blood red.
Low pH and other factors can interfere with the nitrogen fixing bacteria. If the nodules are not active, nitrogen may have to be applied. Fields infested with soybean cyst nematode can appear yellow or have localized yellow areas. Again dig up plants and examine the roots for the cysts. Manganese deficiency will also appear as a yellowing at first.