Сделать цветную область внутри чернобелого background-image

*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

.intro {
  min-height: 50vh;
  position: relative;
  color: white;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
}

.intro::before {
  content: '';
  display: block;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: url(https://picsum.photos/2000/1000) no-repeat center;
  background-size: cover;
  z-index: -1;
}

.intro.grayscale::before {
  filter: grayscale(100%);
}

.intro__content {
  padding: 10px;
  font-size: 30px;
  background-color: rgba(0, 0, 0, 1);
  text-align: center;
}

<div class="intro grayscale">
  <div class="intro__content">С grayscale</div>
</div>
<div class="intro">
  <div class="intro__content">А это без grayscale</div>
</div>

*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

.intro {
  min-height: 100vh;
  position: relative;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  color: white;
}

.intro > * {
  display: flex;
  justify-content: center;
  align-items: center;
}

.intro::before {
  content: '';
  display: block;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: url(https://picsum.photos/2000/1000) no-repeat center;
  background-size: cover;
  z-index: -1;
}

.intro > * + * {
  margin-top: 10px;
}

.intro__content {
  padding: 40px 10px;
  font-size: 30px;
  text-align: center;
  border: 1px solid rgba(0, 0, 0, 0.5);
}

.intro .grayscale {
  backdrop-filter: grayscale(100%);
}

.intro .huerotate {
  backdrop-filter: hue-rotate(120deg);
}

.intro .invert {
  backdrop-filter: invert(70%);
}

.intro .brightness {
  backdrop-filter: brightness(60%);
}

<div class="intro">
  <div class="intro__content grayscale">
  grayscale 100%
  </div>
  <div class="intro__content huerotate">
  hue-rotate 120deg
  </div>
  <div class="intro__content invert">
  invert 70%
  </div>
  <div class="intro__content brightness">
  brightness 60%
  </div>
</div>