Dark Red (#8B0000)

/* Background */
.element {
    background-color: #8B0000;
}

/* Text */
.element {
    color: #8B0000;
}

/* Border */
.element {
    border: 1px solid #8B0000;
}

/* Linear gradient to complementary */
.element {
    background: linear-gradient(
        to right,
        #8B0000,
        #17FFFF
    );
}

/* Radial gradient */
.element {
    background: radial-gradient(
        circle,
        #8B0000,
        #17FFFF
    );
}

// SCSS variable
$dark-red: #8B0000;

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

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