Assuming you're using the Phoenix DDX Scatter plot example here but you can specify the colors you're graphing in the options dictionary and then pass that into your Chart call when charting your graph.

const customColors = [
  '#002159',
  '#03449E',
  '#0967D2',
  '#47A3F3',
  '#BAE3FF'
];
 
var options = {
    colors: customColors,
    width: size.width,
    height: size.height,
    properties: propertyOverrides
  };