This is an extension of the Getting a field value in Read Mode - this will color the field after the value selected.
This css cant be in the banner, but must be "hidden" in a field, the type could be the "Message Custom Field (for view) from the JIRA Toolkit:
<script language=javascript>
var state=AJS.$("#customfield_11102-val").text().trim();
AJS.$("#customfield_11102-val").css("color", "black");
AJS.$("#customfield_11102-val").css("font-weight", "bold");
if (state == 'RED')
{
AJS.$("#customfield_11102-val").css("background-color","#ff0000");
}
if (state == 'AMBER')
{
AJS.$("#customfield_11102-val").css("background-color","gold");
}
if (state == 'GREEN')
{
AJS.$("#customfield_11102-val").css("background-color","#00ff00");
}
if (state == 'WHITE')
{
AJS.$("#customfield_11102-val").css("background-color","white");
}
</script>