InterpolateTextColor - Code.gs

function hexToRgb(hex) {
  var result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
  return result ? {
    r: parseInt(result[1], 16),
    g: parseInt(result[2], 16),
    b: parseInt(result[3], 16)
  } : null;
}

function rgbToHex(color) {
  return "#" + ((1 << 24) + (Math.trunc(color.r) << 16) + (Math.trunc(color.g) << 8) + Math.trunc(color.b)).toString(16).slice(1);
}

function interpolate(from, to, influence) {
  if (influence < 0)
    influence = 0;
  if (influence > 1)
    influence = 1;

  return from + (to - from) * influence;
}

function interpolateRgb(from, to, influence) {
  if (!from || !to)
    return null;

  return {
    r: interpolate(from.r, to.r, influence),
    g: interpolate(from.g, to.g, influence),
    b: interpolate(from.b, to.b, influence)
  }
}

function rgbToString(color) {
  return `(${color.r}, ${color.g}, ${color.b})`;
}

function colorizeSelection(from, to) {
  let rgbFrom = hexToRgb(from);
  let rgbTo = hexToRgb(to);

  let selection = DocumentApp.getActiveDocument().getSelection();

  if (selection) {
    var elements = selection.getRangeElements();
    for (var i = 0; i < elements.length; i++) {
      var element = elements[i];

      if (element.getElement().editAsText) {
        var text = element.getElement().editAsText();

        let fromPosition = element.getStartOffset();
        let toPosition = element.getEndOffsetInclusive();
        let difference = toPosition - fromPosition + 1;

        for (var j = fromPosition; j <= toPosition; j++) {
          let color = rgbToHex(interpolateRgb(rgbFrom, rgbTo, (j - fromPosition) / difference));
          text.setForegroundColor(j, j, color);
        }
      }
    }
  }
}

function openSidebar() {
  DocumentApp.getUi().showSidebar(HtmlService.createHtmlOutputFromFile('Index'));
}

function onOpen() {
  DocumentApp.getUi().createMenu("Colorize").addItem("Run", "openSidebar").addToUi();
}