The Nest Cam going from color to black and white is the night vision feature. Night Vision lets you keep an eye on things, in low light and in the dark. Video will be in black and white whenever Night Vision is on. You can turn this off through the nest app if you don't want to have the Nest Cam go to black and white. It can be changed by going to the Nest app, then going to Settings>Night Vision and set it to off.