I don't know why it doesn't highlight "Circle" and "Color" as valid data types in "Circle.cpp"
REQUIREMENTS:
• no global variables or constants, parameters and return values used for data transfer between functions, descriptive identifier names
• Modify the Circle pi instance variable to be a static member variable
called PI. This can be a constant (Note: Constant static class members
are not explicitly covered in the textbook; however, the const keyword can be used in a static class member declaration and definition to create a constant static class member).
• Add the following functionality for the Circle class:

• Member variable of the enumerated type Color (RED = 1, WHITE =
  2, BLUE = 3). Modify constructors to set/use this additional member
  variable. Use RED as the default color. Include public accessor and
  mutator functions for this additional member variable that get/set the
  enumerated type.
  -All member variables declared private; all member functions declared
  public.
  -Include a copy constructor.
  -Member functions getArea, getDiameter, and getCircumference
  cannot be declared inline
• Friend function displayCircle that takes a single Circle object
  parameter and uses its friend status to descriptively display the
  radius, color (RED, WHITE, or BLUE), area, diameter, and
  circumference to standard output using friendship status access for
  the member variables.
  -Non-friend and non-member function inputCircle that takes no
  parameters and uses descriptive prompts to read Circle instance data
  from user, radius and color, and then returns a Circle object created
  with user input instance data.