Replace history graph colors in lovelace

chart-colors.js

// Add this to your lovelace resources as
// url: /local/chart-colors.js
// type: module

customElements.whenDefined('ha-chart-base').then(() => {

  // Find the HaChartBase class
  const HaChartBase = customElements.get('ha-chart-base');

  // Write a new color list generator
  function getColorList(cnt) {
    let retval = [];
    // green-blue-purple (green is around a hue angle of 150 degrees purple ends around 300 degrees)
    // https://mika-s.github.io/javascript/colors/hsl/2017/12/05/generating-random-colors-in-javascript.html
    const i=150/cnt
    while(cnt--)
      retval.push(Color().hsl(i*cnt+150,80,38));
    return retval;
  }
  
  // Write a new color generator
  // Stole colors from: http://colorbrewer2.org/#type=sequential&scheme=Blues&n=9
  function getColorGenerator(staticColors, startIndex) {
    const palette = [
    "9ecae1","6baed6","4292c6","2171b5","08519c","08306b","c7e9c0","a1d99b","74c476","41ab5d","238b45","006d2c","00441b","bdbdbd","969696","737373","525252","252525","000000"
    ];
    function getColorIndex(idx) {
      // Reuse the color if index too large.
      return Color("#" + palette[idx % palette.length]);
    }
    const colorDict = {};
    let colorIndex = 0;
    if (startIndex > 0) colorIndex = startIndex;
    if (staticColors) {
      Object.keys(staticColors).forEach((c) => {
        const c1 = staticColors[c];
        if (isFinite(c1)) {
          colorDict[c.toLowerCase()] = getColorIndex(c1);
        } else {
          colorDict[c.toLowerCase()] = Color(staticColors[c]);
        }
      });
    }
    // Custom color assign
    function getColor(__, data) {
      let ret;
      const name = data[3];
      if (name === null) return Color().hsl(0, 40, 38);
      if (name === undefined) return Color().hsl(120, 40, 38);
      const name1 = name.toLowerCase();
      if (ret === undefined) {
        ret = colorDict[name1];
      }
      if (ret === undefined) {
        ret = getColorIndex(colorIndex);
        colorIndex++;
        colorDict[name1] = ret;
      }
      return ret;
    }
    return getColor;
  }
  
  // Replace the color list generator in the base class
  HaChartBase.getColorList = getColorList;
  HaChartBase.getColorGenerator = getColorGenerator;

  // Force lovelace to redraw everything
  const  ev = new Event("ll-rebuild", {
      bubbles: true,
      cancelable: false,
      composed: true,
  });
  var root = document.querySelector("home-assistant");
  root = root && root.shadowRoot;
  root = root && root.querySelector("home-assistant-main");
  root = root && root.shadowRoot;
  root = root && root.querySelector("app-drawer-layout partial-panel-resolver");
  root = root && root.shadowRoot || root;
  root = root && root.querySelector("ha-panel-lovelace");
  root = root && root.shadowRoot;
  root = root && root.querySelector("hui-root");
  root = root && root.shadowRoot;
  root = root && root.querySelector("ha-app-layout #view");
  root = root && root.firstElementChild;
  if (root) root.dispatchEvent(ev);
});

The list of hex values after 'palette = ' are the colors I chose.
const palette = [ "9ecae1","6baed6","4292c6","2171b5","08519c","08306b","c7e9c0","a1d99b","74c476","41ab5d","238b45","006d2c","00441b","bdbdbd","969696","737373","525252","252525","000000" ];

I took the colors from http://colorbrewer2.org

Just select your color scheme and click export.

The other place that I changed the colors is in function getColorList(cnt)
It changes the color based on the hsl hue degree and the number of items. I started the function at 150 degrees which is greenish. I wanted the colors to stop at around 300 degrees which is purplish. The total range of degrees is thus 300-150 = 150. I divided the number of degrees by the number of items const i=150/cnt so each item shifts the hue from 150 degrees up to a maximum of 300 degrees.