<div class="redText greenText">What color am I ?</div>

<div class="red">What color is my background ?</div>

<div  id="aquamarine" class="red">What color is my background ?</div>

<div style="background: pink" id="aquamarine" class="red green">What color is my background ?</div>

body {
  font-size: 20px;
  background: gold;
}

.redText {
  color:red;
}

.greenText {
  color:green;
}

#aquamarine {
  background: aquamarine;
}

.red {
  background:red;
}

.green {
  background:green;
}

div #aquamarine {
  font-weight: bolder;
}