First child css

#one {
  width: 500px;
  height: 250px;
  border: 1px solid;
  font-size: 0px;
}
#one #two {
  width: 100%;
  height: 50px;
  background: blue;
}
#one .all {
  width: 250px;
  height: 200px;
  background: green;
  display: inline-block;
}
#one .all:first-child(1) {
  background: gray;
}

<div id="one">
  <div id="two"></div>
  <div class="all"></div>
  <div class="all"></div>
</div>