<fieldset class="rgb">
  <label class="r">R:<input type="number" name="r" min="0" max="255" value="255"></label>
  <label class="g">G:<input type="number" name="g" min="0" max="255" value="255"></label>
  <label class="b">B:<input type="number" name="b" min="0" max="255" value="255"></label>
  <output class="rg">R+G</output>
  <output class="rb">R+B</output>
  <output class="gb">G+B</output>
</fieldset>

body {
  --r: 255;
  --g: 255;
  --b: 255;
  background-color: rgb(var(--r), var(--g), var(--b));
  font-family: Bahnschrift, 'DIN Alternate', 'Franklin Gothic Medium', 'Nimbus Sans Narrow', sans-serif-condensed, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  mix-blend-mode: darken;
}

.r, .g, .b {
  aspect-ratio: 1 / 1;
  border-radius: 50%;
  display: grid;
  font-size: 5cqi;
  mix-blend-mode: color-dodge;
  line-height: 1;
  place-content: center;
  text-align: center;
  input {
    background: #0000;
    border: 2px dotted currentColor;
    color: inherit;
    font-family: inherit;
    font-size: inherit;
    outline-color: currentColor;
    text-align: center;
    &::-webkit-inner-spin-button {
      -webkit-appearance: none;
    }
  }
}

.r {
  background-color: rgb(var(--r), 0, 0);
  color: #FFFC;
  grid-area: 1 / 3 / 6 / 8;
}

.g {
  background-color: rgb(0, var(--g), 0);
  color: #FFFA;
  grid-area: 4 / 1 / 9 / 6;
}

.b {
  background-color: rgb(0, 0, var(--b));
  color: #FFFC;
  grid-area: 4 / 5 / 9 / 10;
}

.rg, .rb, .gb {
  color: #FFF;
  place-content: end;
  text-align: center;
}

.rg {
  grid-area: 4 / 4 / 5 / 5;
}

.rb {
  grid-area: 4 / 6 / 5 / 7;
}

.gb {
  grid-area: 6 / 5 / 7 / 6;
  place-content: center;
}

.rgb {
  all: unset;
  aspect-ratio: 9 / 8;
  container-type: inline-size;
  display: grid;
  font-size: 1.75cqi;
  grid-template-columns: repeat(9, 1fr);
  grid-template-rows: repeat(8, 1fr);
  margin-inline: auto;
  max-width: 800px;
  width: 100%;
}

    const rgb = document.querySelector('.rgb');
rgb.addEventListener('input', e => {
  const N = e.target;
  document.body.style.setProperty(`--${N.name}`, N.value);
})