Bourbon Fundamentals - Tint and Shade Function

This topic is for the [Tint and Shade Function][] exercise in the [Bourbon Fundamentals][] trail. Post any questions, corrections, or pointers you have to share with other Upcase subscribers.
[Tint and Shade Function]: https://exercises.upcase.com/exercises/tint-and-shade-mixin
[Bourbon Fundamentals]: https://thoughtbot.com/upcase/bourbon-fundamentals