Как получить цвет пикселя при нажатии на QGraphicsScene?

Как вариант. Обратите внимание, что я добавил класс GraphicsView, который встроил в форму созданную в Qt Designer. В котором собственно и получаю цвет пикселя. Попробуйте.

import sys
from PyQt5.Qt import *
from PyQt5 import QtWidgets, QtCore, QtGui


#import main_ui
class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(279, 282)
#        MainWindow.setMinimumSize(QtCore.QSize(279, 282))
#        MainWindow.setMaximumSize(QtCore.QSize(279, 282))
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        
        self.widget = QtWidgets.QWidget(self.centralwidget)
        self.widget.setGeometry(QtCore.QRect(10, 10, 258, 213))
        self.widget.setObjectName("widget")
        self.verticalLayout = QtWidgets.QVBoxLayout(self.widget)
        self.verticalLayout.setContentsMargins(0, 0, 0, 0)
        self.verticalLayout.setObjectName("verticalLayout")

#        self.graphicsView = QtWidgets.QGraphicsView(self.widget)
        self.graphicsView = GraphicsView()                                 # +++
        
        self.graphicsView.setObjectName("graphicsView")
        self.verticalLayout.addWidget(self.graphicsView)
        
        self.label = QtWidgets.QLabel(self.widget)
        self.label.setObjectName("label")
        self.verticalLayout.addWidget(self.label)
        
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 279, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)
#        self.show()

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.label.setText(_translate("MainWindow", "Color:"))


class GraphicsView(QGraphicsView):                                        # +++
    clicked = pyqtSignal(str, str)
    
    def mousePressEvent(self, event):
        super(GraphicsView, self).mousePressEvent(event)
        self.setCursor(Qt.CrossCursor)

        # Получить положение мыши на экране
        pos = event.globalPos()
        image = QApplication.primaryScreen().grabWindow(
            int(QApplication.desktop().winId()),
            pos.x() - 6, pos.y() - 6, 13, 13).toImage()
        color = image.pixelColor(6, 6)
        if color.isValid():
            self.clicked.emit('View', color.name())                        

    def mouseReleaseEvent(self, event):
        super(GraphicsView, self).mouseReleaseEvent(event)
        self.setCursor(Qt.ArrowCursor)        
        

class GraphicsScene(QGraphicsScene):
    clicked = pyqtSignal(str, QPointF)

    def mousePressEvent(self, event):
        super(GraphicsScene, self).mousePressEvent(event)
        sp = event.scenePos()
        self.clicked.emit('Scene', sp)
        

class MainWindow(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        
        self.layout = QtWidgets.QHBoxLayout(self.centralwidget)
        self.layout.addWidget(self.widget)

        self.scene = GraphicsScene()
        self.scene.setSceneRect(0, 0, 256, 192)
        self.setMouseTracking(True)
 
        self.graphicsView.setScene(self.scene)

        self.scene.clicked.connect(self.point1)
        self.graphicsView.clicked.connect(self.point1)                  # +++

        self.pen = QtGui.QPen(QtCore.Qt.black)
        self.brush_color = QtGui.QColor("#4AA96C")
        self.scene.addRect(10, 10, 20, 20, self.pen, self.brush_color)
        
        self.ellipse = self.scene.addEllipse(
            QRectF(150, 150, 50, 50), 
            brush=QBrush(QColor(Qt.blue)),
            pen=QPen(QtCore.Qt.red, 5)
        )
        self.ellipse.setFlags(QGraphicsItem.ItemIsSelectable | QGraphicsItem.ItemIsMovable)
        self.ellipse.setZValue(1)
                
        self.pic = self.scene.addPixmap(QPixmap('Ok.png').scaled(60, 60))    
        self.pic.setOffset(100, 50) 

    def point1(self, text, value):
        if text == 'Scene':
            self.label.setText(f'x1 = {value.x()}, y1 = {value.y()}. Color: ')
        if text == 'View':
            self.label.setText(f'{self.label.text()} {value}')
            

if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())