Hi all!
I created example to get pixel color using WINAPI. When I get full screen, it's work fine
My source
Qt Code: 
  1. #ifndef GETCOLOR_H
  2. #define GETCOLOR_H
  3.  
  4. #include <QObject>
  5. #include <QDebug>
  6. #include <windows.h>
  7. #include <QPoint>
  8. #include <QString>
  9.  
  10. class getColor : public QObject
  11. {
  12.     Q_OBJECT
  13. public:
  14.     explicit getColor(QObject *parent = 0);
  15.     Q_INVOKABLE void getMyPixel(int _x, int _y);
  16.  
  17. signals:
  18.     void colorPixel(QString _color);
  19.  
  20. public slots:
  21.     void posApplication(QPoint _pos);
  22.     void posXApplication(int _posX);
  23.     void posYApplication(int _posY);
  24. private:
  25.     int posX;
  26.     int posY;
  27.     QString colorR;
  28.     QString colorG;
  29.     QString colorB;
  30.     QString colorsend;
  31.  
  32.  
  33. };
  34.  
  35. #endif // GETCOLOR_H
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Qt Code: 
  1. #include "getcolor.h"
  2.  
  3. getColor::getColor(QObject *parent) :
  4.     QObject(parent)
  5. {
  6.     posX = posY = 0;
  7.  
  8. }
  9. void getColor::getMyPixel(int _x, int _y)
  10. {
  11. //    HDC dc = GetDC(NULL);
  12.     HDC dc = GetWindowDC(NULL);
  13.     COLORREF color = GetPixel(dc, _x + posX, _y + posY);
  14.  
  15.     int _red = GetRValue(color);
  16.     int _green = GetGValue(color);
  17.     int _blue = GetBValue(color);
  18. //    qDebug()<< "red"<< _red << "green"<<_green << "blue" << _blue;
  19.     ReleaseDC(NULL, dc);
  20.     colorR = QString::number( _red, 16 );
  21.     if(colorR.length() == 1) {
  22.         colorR.insert(0,"0");
  23.     }
  24.     colorG = QString::number( _green, 16 );
  25.     if(colorG.length() == 1) {
  26.         colorG.insert(0,"0");
  27.     }
  28.     colorB = QString::number( _blue, 16 );
  29.     if(colorB.length() == 1) {
  30.         colorB.insert(0,"0");
  31.     }
  32.     colorsend = "#";
  33.     colorsend.append(colorR).append(colorG).append(colorB);
  34.     emit colorPixel(colorsend);
  35. }
  36.  
  37. void getColor::posApplication(QPoint _pos)
  38. {
  39.     posX = _pos.x();
  40.     posY = _pos.y();
  41. }
  42.  
  43. void getColor::posXApplication(int _posX)
  44. {
  45.     posX = _posX;
  46. }
  47.  
  48. void getColor::posYApplication(int _posY)
  49. {
  50.     posY = _posY;
  51. }
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Qt Code: 
  1. #include <QApplication>
  2. #include "qtquick2applicationviewer.h"
  3. #include <QQmlApplicationEngine>
  4. #include <QQmlApplicationEngine>
  5. #include <QQmlEngine>
  6. #include <QString>
  7. #include <QQmlContext> // for setContextProperty
  8. #include "getcolor.h"
  9. #include "qwindow.h"
  10.  
  11. int main(int argc, char *argv[])
  12. {
  13.     QApplication app(argc, argv);
  14.     getColor m_getColor;
  15.     QWindow m_window;
  16.  
  17.     QQmlApplicationEngine engine;
  18.     QtQuick2ApplicationViewer viewer;
  19.     viewer.setSource(QUrl("qrc:///main.qml"));
  20.     viewer.rootContext()->setContextProperty("_getColor",&m_getColor);
  21.     viewer.activeFocusItem();
  22.     viewer.show();
  23.  
  24.     m_getColor.posApplication(viewer.position());
  25.     QObject::connect(&viewer,SIGNAL(xChanged(int)),&m_getColor,SLOT(posXApplication(int)));
  26.     QObject::connect(&viewer,SIGNAL(yChanged(int)),&m_getColor,SLOT(posYApplication(int)));
  27.     return app.exec();
  28. }
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Qt Code: 
  1. import QtQuick 2.3
  2. import QtQuick.Controls 1.2
  3.  
  4. Item {
  5.     id: mainwindows
  6.     visible: true
  7.     width: 640
  8.     height: 580
  9.     property int r : 20
  10.     property color colorPK: "white"
  11.     Rectangle {
  12.         width: 640
  13.         height: 480
  14.  
  15.         gradient: Gradient {
  16.             GradientStop {
  17.                 position: 0.0
  18.                 id: id1
  19.                 SequentialAnimation on color {
  20.                     id: update1
  21.                     loops: Animation.Infinite
  22.                     ColorAnimation { from: "red"; to: "magenta"; duration: 1000 }
  23.                     ColorAnimation { from: "magenta"; to: "blue"; duration: 1000 }
  24.                     ColorAnimation { from: "blue"; to: "cyan"; duration: 1000 }
  25.                     ColorAnimation { from: "cyan"; to: "lime"; duration: 1000 }
  26.                     ColorAnimation { from: "lime"; to: "yellow"; duration: 1000 }
  27.                     ColorAnimation { from: "yellow"; to: "red"; duration: 1000 }
  28.                 }
  29.             }
  30.             GradientStop {
  31.                 position: 1
  32.                 id: id2
  33.                 SequentialAnimation on color {
  34.                     id: update2
  35.                     loops: Animation.Infinite
  36.                     ColorAnimation { from: "yellow"; to: "red"; duration: 1000 }
  37.                     ColorAnimation { from: "red"; to: "magenta"; duration: 1000 }
  38.                     ColorAnimation { from: "magenta"; to: "blue"; duration: 1000 }
  39.                     ColorAnimation { from: "blue"; to: "cyan"; duration: 1000 }
  40.                     ColorAnimation { from: "cyan"; to: "lime"; duration: 1000 }
  41.                     ColorAnimation { from: "lime"; to: "yellow"; duration: 1000 }
  42.                 }
  43.  
  44.             }
  45.         }
  46.  
  47.     }
  48.     Rectangle {
  49.         id: pickerCursor
  50.         x: r; y: r
  51.         width: r*2; height: r*2
  52.         radius: r
  53.         border.color: "black"; border.width: 2
  54.         color: "transparent"
  55.         Rectangle {
  56.             anchors.fill: parent; anchors.margins: 2;
  57.             border.color: "white"; border.width: 1
  58.             radius: width/2
  59.             color: "transparent"
  60.         }
  61.         MouseArea{
  62.             anchors.fill: parent
  63.             id: dragConsolWindowAreaa
  64.             drag.target: parent
  65.             drag.minimumX: 0
  66.             drag.maximumX: mainwindows.width - pickerCursor.width
  67.             drag.minimumY: 0
  68.             drag.maximumY: mainwindows.height - pickerCursor.height
  69.             onPositionChanged: {
  70.  
  71.             }
  72.         }
  73.     }
  74.     Rectangle{
  75.         id: colorReciver
  76.         anchors.bottom: parent.bottom; anchors.bottomMargin:  0
  77.         anchors.horizontalCenter: parent.horizontalCenter
  78.         width: 100
  79.         height: 100
  80.         border.color: "black"
  81.         color: colorPK
  82.     }
  83.  
  84.     Connections{
  85.         target: _getColor
  86.         onColorPixel:{
  87.            colorPK = _color
  88.             console.log(_color)
  89.         }
  90.     }
  91.  
  92.     Timer{
  93.         interval: 30
  94.         repeat: true
  95.         running: true
  96.         onTriggered: {
  97.             _getColor.getMyPixel(pickerCursor.x + pickerCursor.width / 2,pickerCursor.y + pickerCursor.height / 2)
  98.         }
  99.     }
  100. }
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But when I get on QT-QML app, it's not work
code edit
Qt Code: 
  1. #include "getcolor.h"
  2.  
  3. getColor::getColor(QObject *parent) :
  4.     QObject(parent)
  5. {
  6.     posX = posY = 0;
  7.  
  8. }
  9. void getColor::getMyPixel(int _x, int _y)
  10. {
  11. //    HDC dc = GetDC(NULL);
  12.     HWND name = FindWindowA(0,"GetpixelColor");
  13.     HDC dc = GetWindowDC(name);
  14.     COLORREF color = GetPixel(dc, _x, _y);
  15.  
  16.     int _red = GetRValue(color);
  17.     int _green = GetGValue(color);
  18.     int _blue = GetBValue(color);
  19. //    qDebug()<< "red"<< _red << "green"<<_green << "blue" << _blue;
  20.     ReleaseDC(NULL, dc);
  21.     colorR = QString::number( _red, 16 );
  22.     if(colorR.length() == 1) {
  23.         colorR.insert(0,"0");
  24.     }
  25.     colorG = QString::number( _green, 16 );
  26.     if(colorG.length() == 1) {
  27.         colorG.insert(0,"0");
  28.     }
  29.     colorB = QString::number( _blue, 16 );
  30.     if(colorB.length() == 1) {
  31.         colorB.insert(0,"0");
  32.     }
  33.     colorsend = "#";
  34.     colorsend.append(colorR).append(colorG).append(colorB);
  35.     emit colorPixel(colorsend);
  36. }
  37.  
  38. void getColor::posApplication(QPoint _pos)
  39. {
  40.     posX = _pos.x();
  41.     posY = _pos.y();
  42. }
  43.  
  44. void getColor::posXApplication(int _posX)
  45. {
  46.     posX = _posX;
  47. }
  48.  
  49. void getColor::posYApplication(int _posY)
  50. {
  51.     posY = _posY;
  52. }
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I try get color in another app, it's work very fine, but only do not work in QT - QML app. I don't know why?
Please help me to solve this problem.
Thank you for reading!
My project!
https://www.dropbox.com/s/554no9q9eg...Color.rar?dl=0