[doc] Add explicit cast and add unit tests

[sean-g-rae]

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[garydgregory]

Hello @sean-g-rae

This PR needs work. You must make sure that when a change is made to a main class, a corresponding test class fails when the main changes are omitted.

For example, if I apply this PR locally and revert the changes to ColorGroup and run ColorGroupTest, the test is green. This obviously is bad and means that a regression is just waiting to happen.

TY!

[garydgregory]

@sean-g-rae ping 🔔

[sean-g-rae]

@sean-g-rae ping 🔔

Thanks for the feedback! To keep the PR focused, I reverted the non-test change.