Colors in PyQt5 are defined using the method QColor(r, g, b). All colors on the screen are a combination of the red, green and blue values. Every color value should be in the range 0..255. In a QPainter widget you can pass a color to the setBrush method.

QColor example

Related course:
Practice Python with interactive exercises

PyQt5 color example: This example draws a variety of colors in a QPainter widget using the setBrush and QColor methods.

import sys
from PyQt5.QtWidgets import QApplication, QWidget, QMainWindow, QLabel
from PyQt5.QtGui import QPainter, QColor, QPen
from PyQt5.QtGui import QIcon
from PyQt5.QtCore import Qt
import random
class App(QMainWindow):
def __init__(self):
        super().__init__()
        self.title = 'PyQt rectangle colors - pythonspot.com'
        self.left = 10
        self.top = 10
        self.width = 440
        self.height = 280
        self.initUI()
    
    def initUI(self):
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)
        
        # Set window background color
        self.setAutoFillBackground(True)
        p = self.palette()
        p.setColor(self.backgroundRole(), Qt.white)
        self.setPalette(p)
        
        # Add paint widget and paint
        self.m = PaintWidget(self)
        self.m.move(0,0)
        self.m.resize(self.width,self.height)
        
        self.show()
    
class PaintWidget(QWidget):
    def paintEvent(self, event):
        qp = QPainter(self)
        
        qp.setPen(Qt.black)
        size = self.size()
        
        # Colored rectangles
        qp.setBrush(QColor(200, 0, 0))
        qp.drawRect(0, 0, 100, 100)
        
        qp.setBrush(QColor(0, 200, 0))
        qp.drawRect(100, 0, 100, 100)
        
        qp.setBrush(QColor(0, 0, 200))
        qp.drawRect(200, 0, 100, 100)
        
        # Color Effect
        for i in range(0,100):
            qp.setBrush(QColor(i*10, 0, 0))
            qp.drawRect(10*i, 100, 10, 32)
            
            qp.setBrush(QColor(i*10, i*10, 0))
            qp.drawRect(10*i, 100+32, 10, 32)
            
            qp.setBrush(QColor(i*2, i*10, i*1))
            qp.drawRect(10*i, 100+64, 10, 32)
        
if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = App()
    sys.exit(app.exec_())