I tried jQuery code as follows. It showed nothing or had error message [object Undefined].
$("#rgbtxt").keyup(function() {
var value = $(this).val(toString(16));
$("#hextxt").text(value);
}).keyup();
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="rgbhexcolorcode">
<p id="rgb"></p>
RGB <input id="rgbtxt" type="text" value="RGB code">
<p id="hex"></p>
HEXADECIMAL <input id="hextxt" type="text" value="HEXADECIMAL code">
</div>