Given it sounds like you are going to have to create child themes:
"To have multiple versions of Adaptable with subtle differences to match our customer's company colors."
Then ...
"I know I can force themes with cohorts, but that seems to work only for a complete theme change (i.e. Adaptable / Clean)."
Is exactly what you would be doing e.g. your multiple versions (child themes) will need to be named along the lines of adaptable_ford, adaptable_chevy.