Javascript beginner - rgb

Question

I'm trying to make it so that the rgb values entered change the background colour. I have already written the html part.
html:
Red Value<input name="red" type="text">
Green Value<input name="green" type="text">
Blue Value<input name="blue" type="text">
<input name="submit" type="button" value="Submit" onclick="changeBackground();
For the javascript I know it would start with
function changeBackground(){

Answer 1

Try this (see code comments for explanation):
function changeBackground() {
// get values of all input boxes
// getElementsByTagName returns array, you need just first element
var red = document.getElementsByName('red')[0].value;
var green = document.getElementsByName('green')[0].value;
var blue = document.getElementsByName('blue')[0].value;
// create css rgb color value (e.g. rgb(255, 255, 255))
var color = 'rgb(' + red + ', ' + green + ', ' + blue + ')';
// set background color of body
document.body.style.backgroundColor = color;
}
To make it work with background color of text area you need to change just last line. It could look like:
document.getElementById('id_of_my_textarea').style.backgroundColor = color;

Answer 2

Here is a JS fiddle of it working.
Uses
document.forms.RGB.elements
and
document.forms.RGB.style.backgroundColor
https://jsfiddle.net/xy12aten/
intentionally left the code wet so it's easier to read.

Answer 3

This should help get you going
HTML
<form name="RGB">
<p>Red Value<input name="red" value="0" min="0" max="255" required type="number"></p>
<p>Green Value<input name="green" value="0" min="0" max="255" required type="number"></p>
<p>Blue Value<input name="blue" value="0" min="0" max="255" required type="number"></p>
<input name="submit" type="submit" value="Submit">
</form>
JS
var form = document.forms.RGB,
elem = form.elements;
form.onsubmit = function(event){
event.preventDefault()
var red = elem.red.value,
green = elem.green.value,
blue = elem.blue.value;
alert("The color = ("+red+","+green+","+blue+")");
return true;
}
example: https://jsfiddle.net/z3hzt8kf/