上から下へ色がスライドするボタン

html

<a class="btn"><span>パステル。</span></a>

css

.btn span{
  position: relative;
  z-index: 1;
}

.btn {
  width:180px;
  display: block;
  position: relative;
  overflow: hidden;
  margin: 0 auto;
  text-align: center;
  color: #fff;
  font-size: 24px;
  padding: 17px 30px;
  background: #ffcbff;
  box-shadow: 0 3px 3px 0 rgba(0, 0, 0, 0.6);
}

.btn:after {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  height: 140%;
  width: 100%;
  background: #ff77ff;
  transition: all .3s ease-in-out;
  -webkit-transition: all .3s ease-in-out;
  transform: translateY(-95%);
  -webkit-transform: translateY(-95%);
}

.btn:hover:after {
  -webkit-transform: translateY(0%);
  transform: translateY(0%);
}