The syntax generated is not valid in CSS. You are passing in the following:
@color = #FFF
@size = 1px 0px 1px 0px
which in your mixin will generate:
border: 1px 0px 1px 0px solid #FFF;
as CSS. This is not a valid shorthand for borders in CSS. You need something like:
.advancedBorder( @color, @size ){
border-width: @size;
border-color: @color;
border-style: solid;
}