I want every time I click on the 'Armchair' function it changes color

A simple way is to check the current color and change based on it. The code is close to your current one:

function Selecionar(){
    var selecionado = document.getElementById("poltronas");

    // Se estiver amarelo, troca pra verde
    if(selecionado.style.background === "yellow") {
        selecionado.style.background = "green";

    // Senão, troca pra amarelo
    } else {
        selecionado.style.background = "yellow";
    }
}

This will work if the initial color is set to yellow, by JS or inline style:

<div id="poltronas" style="background: yellow"></div>

Without this the initial color would be undefined from the point of view of this script, and the first click would define yellow. From there it would function normally.

If you have more than two colors, you can use an array to store the values and then change according to the amount of click.

Commented example:

/* Define as cores */
const cores = ["yellow", "green", "red", "orange"]

/* Define a posição da cor atual */
let corIndex = 0;

/* Elemento que irá sofrer as alterações */
const poltrona = document.getElementById("poltronas");

function Selecionar(){
  /**
   * Verifica se é uma cor válida
   * Caso não seja, atribui o índice 0
   */
  if (!cores[corIndex]) {
    corIndex = 0
  }
  
  /* Altera a cor e muda o índice para a próxima */
  poltrona.style.setProperty("background", cores[corIndex++])
}

#poltronas {
  height: 100px;
  width: 100px
}

<a href="#" onclick="Selecionar()">Mudar de cor</a>

<div id="poltronas"></div>