doc: Document the signal color parameter

The signal spec now accepts an optional third parameter which is the
color.  Document it so that people know about it.
diff --git a/doc/InteractivePlotter.ipynb b/doc/InteractivePlotter.ipynb
index 3560b7d..f6d0cc2 100644
--- a/doc/InteractivePlotter.ipynb
+++ b/doc/InteractivePlotter.ipynb
@@ -300,7 +300,7 @@
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "ILinePlots can zoom all at the same time.  You can do so using the `group` and `sync_zoom` parameter.  All ILinePlots using the same group name zoom at the same time."
+      "ILinePlots can zoom all at the same time.  You can do so using the `group` and `sync_zoom` parameter.  All ILinePlots using the same group name zoom at the same time.  Note the use of signals with colors."
      ]
     },
     {
@@ -309,7 +309,7 @@
      "input": [
       "trappy.ILinePlot(\n",
       "    trace1,\n",
-      "    signals=[\"cpu_in_power:dynamic_power\", \"cpu_in_power:load1\"],\n",
+      "    signals=[\"cpu_in_power:dynamic_power:18,140,171\", \"cpu_in_power:load1:0xcf,0x36,0x4a\"],\n",
       "    pivot=\"cpus\",\n",
       "    group=\"synchronized\",\n",
       "    sync_zoom=True\n",