Colour is formed by light passing through various layers of a material, with some frequencies being absorbed and others bounced back, the ones that bounce back are what we perceive as colour.
Many factors can determine what bounces back to us, including the direction of light, level of light, atmospheric conditions, as well as the various states of the material its bouncing off.
As you can see in this colour graphic, http://3.bp.blogspot.com/-qxdmWPQSjco/TZWxcZeuPgI/AAAAAAAAIhk/5uDXaKrBOcE/s1600/spectrum.JPG Hazel tones sit next to green and yellow tones.
At night you will be in ether low level light or using artificial lights, when sad your eyes may tear up creating a thin film over the eye. All of these variations can alter what light is bounced out of your eyes tipping the perceived tone between hazels and greens.
If you’re interested this is a great video explaining the same effect on metal surfaces https://youtu.be/NhjiIPohUyw?t=26s