Interesting interpretation. I like it! I was thinking yellow for a brighter future, and your interpretation contrasts it with yellow for an aging past.
I suppose yellow could also be the color of the estate buildings. I don't recall if the family history talks about the stones the estate was built from.
Out of curiosity, I did a google image search on book covers to see how publishers were marketing people's first impressions. There's quite a large variety. Some feature the estate, some feature the people enjoying life at the estate, and some feature both the estate and the people. It seems to be a balanced mix. Which people are featured is interesting: some are couples, some are an individual woman, some are an individual man and some are the ensemble. I was surprised to see one cover that featured the painter only.