Dark Blue (#00008B)

/* Background */
.element {
    background-color: #00008B;
}

/* Text */
.element {
    color: #00008B;
}

/* Border */
.element {
    border: 1px solid #00008B;
}

/* Linear gradient to complementary */
.element {
    background: linear-gradient(
        to right,
        #00008B,
        #FFFF17
    );
}

/* Radial gradient */
.element {
    background: radial-gradient(
        circle,
        #00008B,
        #FFFF17
    );
}

// SCSS variable
$dark-blue: #00008B;

// With RGB channels (useful for rgba() usage)
$dark-blue-r: 0;
$dark-blue-g: 0;
$dark-blue-b: 139;

// Usage
.element {
    background-color: $dark-blue;
    color: rgba($dark-blue-r, $dark-blue-g, $dark-blue-b, 0.8);
}