typedef enum
{
	ShapeType_RECT,
	ShapeType_CIRCLE
} ShapeType;

typedef struct
{
	ShapeType type;
	union
	{
		Rect rect;
		int radius;
	};

	Color color;
	// etc
} Shape;

// So, the usual way of doing dynamic types in C: Enum describing which type it is, a union of the various type-specific values, also values common to both like color.