If null values are provided to a colorscale attribute (e.g., color/z), plotly.js uses gray for color encoding, for example:

https://codepen.io/cpsievert/pen/BbmYoQ
https://community.plot.ly/t/how-to-change-colors-for-na-values-to-gray-in-a-choropleth-map/15746/4

The problem is, there doesn't seem to be an official to control this in a way that is decoupled from color/z values themselves. Ideally, you would be able to control this from the colorscale definition, perhaps like this https://codepen.io/cpsievert/pen/xBPYZM