const char esc = (char)0x1b;

// not all terms support this; in case not, a muxer like tmux or screen will approximate it
public string TrueColorEscaped(string obj, int r, int g, int b) {
  // <ESC>[38;2;{r};{g};{b}m sets a true color color
  // <ESC>[0m resets color
  return String.Format("{0}[38;2;{1};{2};{3}m{4}{0}[0m", esc, r, g, b, obj);
}

foreach (var i in Enumerable.Range(0, 255))
  Console.WriteLine(TrueColorEscaped("Hello world", i, 0, 0));