1). Sensor colors are based on the US EPA PM2.5 AQI. Here is a link to a community article that shows those AQI colors. It is also important to note that PurpleAir sensors do not use a static color for each AQI level, but rather a slight gradient.
2.) As of right now, there is no way for users to change the colors or AQIs displayed on their PurpleAir devices. This is a good suggestion, and I will pass it along to our development team.
Please let us know if you need anything else.