Yellowing is a sign of nitrogen depletion. Nitrogen is used in decomposition, so it’s not surprising with all the fresh stump grinding material in the soil. You will probably see mushrooms at some point as well. Nitrogen also encourages leaf growth, which you do not want to encourage at this time. All fertilizer blends are described using numbers to represent the percentage of materials in the following order – Nitrogen, phosphorus, potassium or N-P-K. Fertilize with a low nitrogen fall/ winter blend (6-12-18 or equal) now and in the spring when things start to yellow give regular feedings with high nitrogen (24-8-16). The numbers will vary according to manufacturer, but the nutrients are always listed in the same order. Better, more costly blends have more in-soluble nitrogen which releases slower and lasts longer, organic being the slowest release and least potent (6-6-6 or equal). You will need lots of nitrogen until decomposition is complete in a few years. No big secret – when it’s yellowing, add fertilizer.