The ToColorName returns string with one of the six colors: black, blue, green, yellow, red, white converted from colorNum parameter. You can use the ToColorName function to convert value obtained from color sensor by method ReadColorSensor with mode set to ColorDetect.

If the mode ColorDetect is chosen, then the returned values will be a number between 1 and 6 which represent the following colors: black, blue, green, yellow, red, white.

txtColor = nxt.ToColorName( colorNum );

function OnStart()
{
  nxt = app.CreateNxt();
  app.SetMenu( "Connect,Sense Color" );
}
 
function OnMenu( name )
{
  if( name=="Connect" )
    nxt.ShowDevices();
 
  else if( name=="Sense Color" )
  {
    clr = nxt.ReadColorSensor( 3, "ColorDetect" );
    app.ShowPopup( nxt.ToColorName( clr ) );
  }
}