--> From OpenCV Basics folder you may learn about functions and techniques that opencv offers and after that you can apply that according your computer vision use cases. from here you can learn reading of image/videos/webcame, convert images into grayscale, blur, canny, image dialation..(thik liness), Erorde image, resizing and cropping images, draw shapes(circle,rectangle,lines,etc..) and write text on images, stacking images/ display multiple images, color detection using tracker and cv2 then shape detection, and face detection using haarcascade classifier.
Apply knowledge of opencv and use color tracker you can change color as per your need, the ouput of virtula painting will look like as below..
Use Yolo4 with single frame/image to detect multiple object from it. as well as detect objects from video.. output will be look like belows..
Video result: