Position div relative to fixed div with variable content

Hello! I am trying to position my "color" div exactly 10px below my fixed "header" which has variable content depending on width of device.

HTML

<header id="header">
   <div id="logo"> <a href="/" class="nonblocklink"><img alt="A logo." src="images/logo.png"></a> </div>
   <div id="nav">
      <ul>
         <li id="projects">
            PROJECTS
            <ul>
               <li id="one"> <a href="one/" class="blocklink" target="_self">ONE</a> </li>
            </ul>
         </li>
      </ul>
   </div>
</header>
<div id="color">
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div> 
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div class="color"></div>
   <div...

CSS

a.nonblocklink {
	text-decoration: none;
	color: inherit;
	display: inline-block;
}

a.blocklink {
	text-decoration: none;
	color: inherit;
	display: block;
}

#header {
	background-color: white;
	width: 100%;
	position: fixed;
	margin: 0px;
	padding: 0px;
	padding-top: 10px;
	top: 0;
	left: 0;
	text-align: center;
	z-index: 10;
}

#logo img {
	width: calc(100% - 20px);
}

#logo img:hover {
	-webkit-filter: invert(100%) !important;
}

#nav {
	width: 100%;
	background-color: white;
}

#projects {
	display: inline-block;
}

#nav ul {
	font-family: Arial;
	font-size: 15px;
	font-weight: bold;
	color: #000000;
	list-style-type: none;
	text-align: center;
	margin: auto;
	padding-top: 6px;
	padding-right: 0px;
	padding-bottom: 10px;
	padding-left: 0px;
}

#nav ul ul {
	width: calc(100% - 20px);
	list-style-type: none;
	font-weight: normal;
	display: none;
}

#nav ul li:hover>ul {
	display: block;
	position: absolute;
	text-align: center;
	margin: 0 auto;
	padding-top: 10px;
	left: 0;
	right: 0;
}

#one {
	padding-top: 10px;
	padding-bottom: 10px;
	border: 1px solid black;
	color: #000000;
	background-color: white;
}

#one:hover {
	background-color: black;
	color: white;
}

#footer {
	width: 100%;
	background-color: white;
	position: fixed;
	margin: 0px;
	bottom: 0;
	left: 0;
	text-align: center;
	z-index: 11;
}

#footer-nav {
	width: 100%;
}

#info {
	display: inline-block;
	padding-top: 10px;
}

#footer-nav ul {
	font-family: Arial;
	font-size: 15px;
	font-weight: bold;
	color: #000000;
	list-style-type: none;
	text-align: center;
	margin: auto;
	padding-top: 0px;
	padding-right: 0px;
	padding-bottom: 10px;
	padding-left: 0px;
}

#footer-nav ul ul {
	width: calc(100% - 20px);
	list-style-type: none;
	font-weight: normal;
	display: none;
}

#footer-nav ul li:hover>ul {
	display: block;
	position: absolute;
	bottom: 100%;
	text-align: center;
	margin: 0 auto;
	left: 0;
	right: 0;
	padding-bottom: 0;
}

#email {
	padding-top: 10px;
	padding-bottom: 10px;
	border: 1px...

JavaScript

var colors = ['#d24bd1', '#d44d51', '#dd465c', '#c648d2', '#c046d6', '#d9d5df', '#43dbdb', '#44cfd9', '#d7f4d5', '#d4f753', '#c2ff42', '#49fdff', '#4641f2', '#d0657b', '#e4da5b', '#97f7ec', '#e960eb', '#82e00a', '#1ab60d', '#efb36e', '#07ef05', '#fa86eb', '#e5ee8e', '#ef92e8', '#4b9bf7', '#76f567', '#d3fee5', '#f5dae8', '#e6c9e7', '#e1f0f9', '#f9f3e1', '#77f1e6', '#f2f4e4', '#e1f0f9', '#f9f3e1', '#77f1e6', '#e4f45d', '#e4f45d', '#e4f45d', '#e4f45d', '#d4ffdd', '#fc30fa', '#e6f2e0', '#f6dfc6', '#df51ce', '#51f4d3', '#47747b', '#585a7f', '#7a7c45', '#dc71d4', '#FFFFFF', '#FFFFFF', '#FFFFFF', '#FFFFFF'];
    var boxes = document.querySelectorAll(".color");

    for (i = 0; i < boxes.length; i++) {
        boxes[i].style.backgroundColor = colors[Math.floor(Math.random() * colors.length)];
        boxes[i].style.width = '100%';
        boxes[i].style.height = '10px';
        boxes[i].style.display = 'block';
        boxes[i].style.margin = '0 auto'; }