I would like to have a promt or some type of notice that when a user changes
the information in cell B15 that it might be necessary to change the
information in B25.

Ideally, I would want a popup box/notice that says "You've changed the
information within, would you like to change XX as well?"

Realistically, I would settle for a simple change in cell colour or
something similar to indicate that more changes may be needed.

Does anyone have any ideas? Thanks!