I don't understand where to set the values of $color...

I created the mixin, but I'm not sure where to set the values of the variable $color. First I thought the value for that parameter has to be set inside the call of the mixin, but the challenge kept on asking for those values. So, I tried adding it in the parenthesis (after $color: 1px solid), although I knew that was kind of vague. I trief to set the values in the border declaration using border: 1px solid; But it asked me to set the values for $color to 1px solid. Then I created a call below, just to see what happened. Nothing. Maybe somebody knows the answer, because even after watching the previous videos three times, I didn't get it. I might be looking in the wrong direction...

Thank you so much!

@mixin square ($size, $color) {
  height: $size;
  width: $size;
  border: $color;
}

@include square ($color: 1px solid);