JavaScript validation is not secure as anybody can change what your script does in the browser. Using it for enhancing the visual experience is ok though.
var textBox = document.getElementById("myTextBox");
var textLength = textBox.value.length;
if(textLength > 5)
{
//red
textBox.style.backgroundColor = "#FF0000";
}
else
{
//green
textBox.style.backgroundColor = "#00FF00";
}