codice:
function chgColor()
{
	if (currColor == "yellow")
	{
		document.getElementById("scritta").style.color="red";
		currColor = "red";
	}
	else if (currColor == "red")
	{
		document.getElementById("scritta").style.color="green";
		currColor = "green";
	}
	else if (currColor == "green")
	{
		document.getElementById("scritta").style.color="yellow";
		currColor = "yellow";
	}

}

oppure
codice:
var colori = ["black", "yellow", "red", "green", "blue", "maroon"];
var indice_colore = 0;
function chgColor()
{
	indice_colore++;
	if(indice_colore >= colori.length ) indice_colore = 0;
	document.getElementById("scritta").style.color = colori[indice_colore];
}