Yellowing an Etrog on Shabbat

Yellowing etrogim: Is there a prohibition to place an etrog among apples or other fruits in order for it to cause it to turn yellow, especially on Shabbat, which could potentially involve the issues of 'preparation' and 'muktzeh'? Additionally, since it becomes ready for eating through this process, could there be a concern of cooking?

Shalom!
It is permitted to cause an esrog to turn yellow on a weekday — the yellowing by apples or other fruits is not just a color change, rather, it accelerates the ripening of the esrog, and it is no longer 'unripe'.
Regarding yellowing on Shabbat — our teacher Rabbi Amram Fried ruled that it is permitted.
There is no prohibition to prepare food on Shabbat if one does not perform a 'melacha' (forbidden labor) for this purpose. And there is no prohibition of ‘fixing’ food on Shabbos.
This does not involve the forbidden labor of 'coloring', since the fruit itself ripens and changes color, and no external color is applied to it. And it is not the labor of 'cooking', which involves repairing food by applying heat.
However, on Shabbos, it is forbidden to move an esrog that one wants to use for the mitzvah, since a person is careful not to use it for any purpose, so that it will not be damaged and disqualified. Therefore, it is considered 'muktzeh machmas chisron kis' (due to monetary loss).

Shulchan Aruch, section Orach Chaim, chapters 308, 318, 320.
And see Tshuvos ve-Hanhagos, vol. V, chapter 212.