This function mutates :VegaVisualization, modifying the colors for the group or fill properties of the visualization. The palette argument supports:
* Tuple{AbstractString,Int64}: A ColorBrewer theme and number of color levels desired
* AbstractString: A string representing a color the browser understands (i.e. "Pink", "#FFF")
* Array: An array of colors (i.e. ["Red", "Green"])