animation-delayにマイナス値を指定したのサンプル

.foo,
.bar {
  width: 200px;
  height: 200px;
  border-radius: 50%;
  background: blue;
}

.foo {
  animation: color-change 10s;
}

.bar {
  animation: color-change 10s -5s;
}

@keyframes color-change {
  0% {
    background: blue;
  }
  50% {
    background: yellow;
  }
  100% {
    background: red;
  }
}