Changing text color in a label

I have an asp:label with some default text in gray. I have a javascript script that makes this default text disappear when the label getas focus.
When the label gets focus I want to display the entered text in black, not grey.
How would I do this?
Many thanks.
Here is the script:
<script type="text/javascript">
function IsValid(_this, Defval) {
    if (_this.value == Defval) {
        _this.value = "";
    }
}
</script>
Which is invoked with:
tbLimitEntryPrice1.Attributes.Add("onfocus", "IsValid(this, \"Limit Entry Price\")");


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