Yellowing stems and leaves on geranium cuttings can have many different causes including water issues, temperature extremes, poor health, or nutrient deficiencies. Here’s a few possibilities: 1) Under or over-watering: lower leaves start yellowing first. 2) Exposure to cold temperatures (below 45°F or 7°C). 3) Unhealthy plant. 4) Nutrient deficiency in the potting mix. You could try repotting it in new potting mix or adding a slow-release organic fertilizer (follow instructions on label).