HTML
<section class="robotron">
<img class="robo" id="robotron" src="img/robotron.png" alt="Robotron">
<button onclick="mudarImagem ()">Cor Robotron</button>
<figcaption class="titulo">ROBOTRON <br>2000</figcaption>
</section>
JS
var imagens = ["img/Robotron 2000 - Amarelo.png", "img/ Robotron 2000 - Azul.png", "img/Robotron 2000 - Branco.png", "img/Robotron 2000 - Preto.png", "img/Robotron 2000 - Rosa.png", "img/Robotron 2000 - Vermelho.png"];
var indice = 0;
function mudarImagem () {
var img = document.getElementById("robotron");
indice = (indice + 1) % imagens.length;
img.src = imagens[indice];
}