Generated by SassMeister.com.

SassMeister-input-HTML.html

<ul>
  <li>a</li>
  <li>b</li>
  <li>c</li>
  <li>d</li>
  <li>e</li>
  <li>f</li>
  <li>g</li>
  <li>h</li>
  <li>i</li>
  <li>j</li>
  <li>k</li>
  <li>l</li>
</ul>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

$colors: red, orange, yellow, green, blue, purple;
$repeat: 20;

@for $i from 1 through $repeat {
    li:nth-child(#{length($colors)}n+#{$i}) {
        background: lighten(nth($colors, random(length($colors))), 20%);
    }
}

li {
    list-style: none;
    padding: 1em;
}

SassMeister-output.css

li:nth-child(6n+1) {
  background: #ffff66;
}

li:nth-child(6n+2) {
  background: #e600e6;
}

li:nth-child(6n+3) {
  background: #ffff66;
}

li:nth-child(6n+4) {
  background: #6666ff;
}

li:nth-child(6n+5) {
  background: #ff6666;
}

li:nth-child(6n+6) {
  background: #ffff66;
}

li:nth-child(6n+7) {
  background: #ff6666;
}

li:nth-child(6n+8) {
  background: #00e600;
}

li:nth-child(6n+9) {
  background: #ff6666;
}

li:nth-child(6n+10) {
  background: #6666ff;
}

li:nth-child(6n+11) {
  background: #6666ff;
}

li:nth-child(6n+12) {
  background: #ffff66;
}

li:nth-child(6n+13) {
  background: #ff6666;
}

li:nth-child(6n+14) {
  background: #00e600;
}

li:nth-child(6n+15) {
  background: #e600e6;
}

li:nth-child(6n+16) {
  background: #ffc966;
}

li:nth-child(6n+17) {
  background: #00e600;
}

li:nth-child(6n+18) {
  background: #6666ff;
}

li:nth-child(6n+19) {
  background: #6666ff;
}

li:nth-child(6n+20) {
  background: #ff6666;
}

li {
  list-style: none;
  padding: 1em;
}

SassMeister-rendered.html

<ul>
  <li>a</li>
  <li>b</li>
  <li>c</li>
  <li>d</li>
  <li>e</li>
  <li>f</li>
  <li>g</li>
  <li>h</li>
  <li>i</li>
  <li>j</li>
  <li>k</li>
  <li>l</li>
</ul>