The yellowing could be for any of a number of reasons, such as lack of water, over-watering, lack of nitrogen or lack of sun light penetration to the yellowing leaves.
Let the plant go into dormancy outside before storing inside with care so it does not see low temperature below 25F to avoid freeze damage.
Over-wintering in dormant state requires that the plant be stored in place wher temperature will stay between 32F to 45F. Temperature higher than 45F can break dormancy. Temperatures below -5F can cause freeze damage.