Crimson Red (#990000)

/* Background */
.element {
    background-color: #990000;
}

/* Text */
.element {
    color: #990000;
}

/* Border */
.element {
    border: 1px solid #990000;
}

/* Linear gradient to complementary */
.element {
    background: linear-gradient(
        to right,
        #990000,
        #33FFFF
    );
}

/* Radial gradient */
.element {
    background: radial-gradient(
        circle,
        #990000,
        #33FFFF
    );
}

// SCSS variable
$crimson-red: #990000;

// With RGB channels (useful for rgba() usage)
$crimson-red-r: 153;
$crimson-red-g: 0;
$crimson-red-b: 0;

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