Is there a standard color palette (or most commonly used in publications and other materials) for the spectral indices?
There is no standard color palette for spectral indices. The most important is to represent the indices in a way that can understandable to someone looking at the map. Choosing a color palette is a matter of personal preference, some people prefer a range of strong tone with various (different) colors, others may prefer a range of same color (light to dark green), or a range of two colors (yellow/white to green, for example). It is recommended to choose a color that can be easily recognized on the map.