js:
var colorize = function(text) {
  var delimiter = "_",
      parts = text.split(delimiter),
      firstPart = $("<span>", { class: 'red', html: parts[0] }),
      secondPart = $("<span>", { class: 'blue', html: text.split(parts[0] + delimiter)[1] });
  return $("<div>").append(firstPart).append(delimiter).append(secondPart).html();
}
編集:次のようなjQuery要素を受け取るヘルパー関数を利用することもできます。
var colorizeElements = function($elements) {
  // $elements has to be something like $("td.classname")
  $elements.each(function() {
    $(this).html(colorize($(this).html()));
  });
};
編集:作業中のjsFiddle:http ://jsfiddle.net/tGV54/1/
編集:Kristoffersコードに基づく:
String.prototype.colorize = function () {
    var regex = new RegExp('[^_]+', 'gi'),
        color = $.extend(['blue', 'red'], arguments || []),
        index = 0;
    return this.replace(regex, function(matched) {
        return '<span style="color: ' + color[index++] + ';">' + matched + '</span>';
    });
};
$.fn.customColorize = function () {
    var args = arguments,
        delimiter = "<br>";
    return this.each(function () {
        var splitetElements = $.map(this.innerHTML.split(delimiter), function(s) { return s.colorize(); });
        this.innerHTML = splitetElements.join(delimiter);
    });
};