You will be able to find Cor Lapiz near Mt Hula, where you will see a yellow item, so simply point your camera to automatically photograph it.
Sweet Flower will also work as a very common item that is easily found in Mondstadt, Liyue, and elsewhere.
Berries also count and are also very easily found.
Please note that even though there are five color cards listed on the event page, the camera will randomly extract colors from the items it photographs, and you will need at least one of each color in order to get the Ji Tong reward.
Now that you know where to find yellow items in Genshin Impact, you are ready to start the next challenge, so go ahead.