Once the shells are cooled, flip over every other shell—match up same sized/shaped shells as best as possible. Pipe the buttercream filling onto the flat-side up shells. Top the macarons with the remaining shells rounded side up; lightly press to sandwich them. Keep refrigerated until ready to serve; the macarons actually taste best the next day when all the flavors have melded. You may freeze macarons in an airtight container between sheets of wax paper.
Notes: The Wilton white icing tint is just that; a food grade titanium oxide based pigment which will whiten/brighten icings and batters. The use of the white icing tint is optional, but because of the natural color of the almond meal, the batter will result in an off-white shade of almond yellow or ivory if not used. I chose to add the colorant as I wanted the striking contrast of white to red in the final product.
In baking, whether or not the white shells are tinted, the edges can turn tan in color. If this occurs before the baking time is up, and the shells are not yet set, try reducing the oven temperature to 325° and go a little longer in the oven if needed.