Properties, style tab, fourth one down. Anticipating possible issues, that style STACKS the areas as I recall. So you may need to subtract the first expression from the second expression in the second expression. The first line will have color underneath it, and you may want it to be white, so that ONLY the area between the lines is colored. I suspect you could just make the first color on the Colors tab white, and that would work. If not, a background color expression on the first expression of white().
