When you stare into a white light source for a period of time and then shift your gaze to another surface, you may perceive an afterimage or an optical illusion. The phenomenon you describe, seeing a green or pink spot instead of white, can be explained by the way our eyes and visual system perceive and process light.
The human visual system has cells in the retina called photoreceptors that are responsible for detecting light. These photoreceptors are of two types: cones and rods. Cones are responsible for color vision and function best in bright light, while rods are more sensitive to dim light but do not perceive color.
When you stare at a bright white light, your cone cells become strongly stimulated. They send signals to your brain indicating the presence of intense light. However, prolonged exposure to such intense light can lead to temporary desensitization or fatigue of the cone cells. This can result in a phenomenon called "bleaching" or "adaptation."
When you shift your gaze to a neutral surface after staring at the white light, the cone cells that were fatigued or desensitized may respond less strongly than the surrounding cells. This relative difference in response leads to a perception of an afterimage that appears in a complementary color to the light you were originally looking at.
In the case of a green afterimage, it is often because the cone cells responsible for perceiving red light were fatigued, leading to a relatively stronger response from the cone cells responsible for perceiving green light. This results in a perception of green when looking at another surface.
Similarly, a pink afterimage may occur when the cone cells responsible for perceiving green light were fatigued, leading to a relatively stronger response from the cone cells responsible for perceiving red light. The combination of remaining sensitivity to red and the lack of sensitivity to green can create a perception of pink.
It's important to note that individual variations in color perception and differences in the specific light sources can also influence the exact colors perceived in afterimages.