The part I want to select doesn't have a unique color area in the selection areas.

Defining the perfect area borders is somewhat subjective, though even imperfect Selection Areas are a big time-saver. With more complex objects the delineation between different objects and materials can get hard to define and use cases difficult to predict. We use a combination of automated tools and human intuition to decide which bits get their own unique key colors, so you may need to do some minor trimming or combining of your masks to get exactly the selections you desire based on the specific needs of your project, but on the whole, Selection Areas provide enormous time savings in just about every case.