Why do people have red eyes when they take pictures, while most dogs have green or blue eyes?

The reason for blushing is that the eyes are in a dark room with little light. In order to absorb the light, the pupils dilate, so that when the flashlight hits the eyes, the camera captures the blood vessels on the retina inside the pupils, thus making the eyes in the photos appear red. So photos taken in dark places usually have red eyes.

Because the dog's eyes absorb the red part of light more strongly than other parts of light, the dog's eyes that reflect back to our eyes are green.

Under normal circumstances (sufficient natural light, no flash is needed), red eyes will not appear in human eyes and dogs' eyes, because the pupils are contracted at this time, and the scattered natural light will not cause the lens to focus on reflecting blood vessels.