How to force Swagger Codegen to generate an enum which is not referenced from any other object?

Finally I've solved the issue by moving the declaration of ColorType into top-level components section. Prior to fix I had only securitySchemes declaration under the section:

components:
  securitySchemes:
    bearer:
      type: http
      scheme: bearer

I've changed it to

components:
  schemas:
    ColorType:
      type: string
      enum:
        - ACCENT_1
        - ACCENT_2
        - BACKGROUND_1
        - BACKGROUND_2
        - TEXT
        - SOLID_BUTTON
        - OUTLINE_BUTTON
  securitySchemes:
    bearer:
      type: http
      scheme: bearer

This gave me desired Java enum among generated classes:

public enum ColorType {
  ACCENT_1("ACCENT_1"),
  ACCENT_2("ACCENT_2"),
  BACKGROUND_1("BACKGROUND_1"),
  BACKGROUND_2("BACKGROUND_2"),
  TEXT("TEXT"), 
  SOLID_BUTTON("SOLID_BUTTON"),
  OUTLINE_BUTTON("OUTLINE_BUTTON");

  private String value;

  ColorType(String value) {
    this.value = value;
  }

  @Override
  @JsonValue
  public String toString() {
    return String.valueOf(value);
  }

  @JsonCreator
  public static ColorType fromValue(String value) {
    for (ColorType b : ColorType.values()) {
      if (b.value.equals(value)) {
        return b;
      }
    }
    throw new IllegalArgumentException("Unexpected value '" + value + "'");
  }
}