Assuming that Involved is Bound to a Field in your underlying Table, and that the Form is in Single View (which I assume to be true, considering the question about a Tabbed Control) you need to place the same code in the Form_Current event, as well as the OnChange event. The same thing would apply to the formatting of the Tabbed Control.
Code pretty much has to be in the Form_Current event anytime you want formatting changes to be Record-specific.
Doing formatting to Continuous or Datasheet View Forms is limited, in most cases, to using Continuous Formatting from the Menu/Ribbon, and has a limited set of Options.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007