Add a new Rule to the Ruleset.  Call it “Favors”, make the rule “Trading” a subrule of it, and give it the following text:

Each Student has a GNDT field “Favour”, which may name one other Student or idle Student. If a Student has a non-blank Favour field, they are considered to “owe a Favour” to the Student or idle Student named in it. If two Students owe each other Favors, any Student may blank both their Favor fields. A Student with a blank Favor field may set their Favor field to another Student at any time. A Student may blank any Favour field that contains their own name, at any time.

If a Game Action is a Favor Action from a Student (the Agent), any other Student with a blank Favor may set their Favor to the Agent to take that action, and the Student to whom the Agent owes a Favor may blank the Agent’s Favor field to take that action.  No Student may take more than one Favor Action with the same Agent in a day or in any given 6-hour period.

Reword the rule “Trading” as follows:

If a Student (the “Trader”) wants to borrow a Pen that belongs to another Student (the “Tradee”), and if both the Trader and Tradee have a Pen Colour, then the Trader may swap the Pen Colour values of the Trader and the Tradee as a Favor Action from the Tradee .