Using JS To Change Color Of The Contents Of A Span Tag

Javascript is not my strong point but I have a little problem with a
function.

I have ASP code that displays form variables for x number of users.
Depending on the radio button they click, I highlite some text in red
over another form variable to show that it's mandatory. I use a <span>
tag for the text and a Javascript function to change the text color of
that user's mandatory field.

My Javascript call passes (i) to show which user I'm talking about but
the problem is I don't know how to change that user's text color.
Here's my function

<script language="JavaScript">
<!--
function ChangeTextColor(num){
membertype_1.style.color='red'
memberno_1.style.color='black'
lastname_1.style.color='black'
}

//-->
</script>

As you see, I'm not using the parameter "num" so it only works for the
first user. I need to know how to make it dynamic...I've tried

membertype_[+num+].style.color='red'

but that doesn't work. Anyone know the syntax?