Triangulo via CSS

Fazer um triângulo via CSS

HTML

<div class="color">
    <div></div>
</div>

CSS

body{
    background:gray;
}
.color{
    width:40px;
    height:40px;
    background:white;
}
.color > div{
    border-right: 40px solid black;
    border-top: 40px solid transparent;
}