Question

I have a header box including border and padding and background color for that box,
can I change the background color only for the padded region after the border and then the same background color for the rest of the width (i.e. grey in the given code)?

Just a pinch of the code where I want the padded background color:

nav {
    margin:0px auto;
    width:100%;
    height:50px;
    background-color:grey;
    float:left;
    padding:10px;
    border:2px solid red;
}

Answer 1

Another option with pure CSS would be something like this:

nav {
    margin: 0px auto;
    width: 100%;
    height: 50px;
    background-color: white;
    float: left;
    padding: 10px;
    border: 2px solid red;
    position: relative;
    z-index: 10;
}

nav:after {
    background-color: grey;
    content: '';
    display: block;
    position: absolute;
    top: 10px;
    left: 10px;
    right: 10px;
    bottom: 10px;
    z-index: -1;
}

<nav>Some text or anything</nav>

Demo here