The second parameter of hsl() defines the saturation. Less saturation mean less color. 0% is completely gray:
<h2 style="background-color:hsl(0, 100%, 50%);"> <h2 style="background-color:hsl(0, 80%, 50%);"> <h2 style="background-color:hsl(0, 60%, 50%);"> <h2 style="background-color:hsl(0, 40%, 50%);"> <h2 style="background-color:hsl(0, 20%, 50%);"> <h2 style="background-color:hsl(0, 0%, 50%);">