<div class="box">
  <p>左上</p>
  <div class="example ex01"></div>
</div>

<div class="box">
  <p>右上</p>
  <div class="example ex02"></div>
</div>

<div class="box">
  <p>右下</p>
  <div class="example ex03"></div>
</div>

<div class="box">
  <p>左下</p>
  <div class="example ex04"></div>
</div>

* {
  box-sizing: border-box;
}
body {
  min-height: 100vh;
  margin: 0;
  padding: 0;
  display: flex;
  justify-content: center;
  align-items: center;
}
.box {
  text-align: center;
}
.box:not(:last-of-type) {
  margin-right: 5%;
}
.example {
  width: 0;
  height: 0;
}
.example.ex01 {
  border-style: solid;
  border-width: 30px 30px 0 0;
  border-color: #a0dde2 transparent transparent transparent;
}
.example.ex02 {
  border-style: solid;
  border-width: 0 30px 30px 0;
  border-color: transparent #e2a0b4 transparent transparent;
}
.example.ex03 {
  border-style: solid;
  border-width: 0 0 30px 30px;
  border-color: transparent transparent #eec496 transparent;
}
.example.ex04 {
  border-style: solid;
  border-width: 30px 0 0 30px;
  border-color: transparent transparent transparent #a0e2c2;
}