When I adopted my first puppy Joy, she was cream/white colored. A year later, she was the same color. I then adopted her half-sister Star, who was a dark red/gold color. Within a year, Joy had gotten darker and Star had gotten lighter—so they met in the middle. They became and still are apricot/blonde and now look like twins! I'm blonde also so perhaps it's because our spirits are so connected! Just curious how often this happens.
Many breeds have coats that change over time. Puppies will generally be a different color than the adult dog. Medications, sunlight, nutritional status and skin disease can all be factors in changing and dog's coat color. It might be that your puppies had different nutritional backgrounds and when they were brought together their nutritional and environmental influences matched. Coat color can change without medical or skin disease being present.
However, if you notice any signs of skin disease, hair loss, irritation or discomfort, then a trip to the veterinarian is best. There are breed characteristics that might influence hair color as well. Here's a list of causes of pigment and color changes in skin and coat of dogs that may be helpful.
Sometimes changes in color is just the progression of that dog's pigment in the hair. Thanks for sharing your photos of Joy & Star.