Minimal ROS rqt plugin that stores a color setting

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(qt_color_setting_test)

find_package(ament_cmake REQUIRED)
find_package(pluginlib REQUIRED)
find_package(rqt_gui_cpp REQUIRED)
find_package(Qt5Widgets REQUIRED)

add_library(${PROJECT_NAME} SHARED src/color_setting.cpp)
target_include_directories(${PROJECT_NAME} PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>)
ament_target_dependencies(${PROJECT_NAME}
  rqt_gui_cpp
  Qt5Widgets)
pluginlib_export_plugin_description_file(rqt_gui plugin.xml)
install(
  TARGETS ${PROJECT_NAME}
  EXPORT export_${PROJECT_NAME}
  LIBRARY DESTINATION lib
)
ament_package()

color_setting.cpp

#include "color_setting.hpp"
#include <iostream>

void ColorSetting::saveSettings(
  qt_gui_cpp::Settings &,
  qt_gui_cpp::Settings & instance_settings) const
{
  instance_settings.setValue("color", QColor("green"));
}

#include "pluginlib/class_list_macros.hpp"
PLUGINLIB_EXPORT_CLASS(ColorSetting, rqt_gui_cpp::Plugin)

color_setting.hpp

#pragma once
#include "rqt_gui_cpp/plugin.h"
class ColorSetting : public rqt_gui_cpp::Plugin
{
public:
  ColorSetting() = default;
  ~ColorSetting() = default;
  void saveSettings(
    qt_gui_cpp::Settings &,
    qt_gui_cpp::Settings & instanceSettings) const override;
};

plugin.xml

<library path="qt_color_setting_test">
    <class type="ColorSetting" base_class_type="rqt_gui_cpp::Plugin" />
</library>