To ensure macarons are the same size, you can trace 1-inch circles with a pencil on parchment paper, then flip the paper and use the circles as a guide when piping.
If you want to dye the macaron batter, add food coloring to egg whites in step 2; make the mixture a few shades darker than desired as baking will lighten the color. Use colors in powders or gel form - liquid food coloring may add too much moisture to the egg mixture and change the consistency.
Make sure you bake only 1 baking sheet at a time.
If you want to dye the buttercream filling, add to the butter mixture in step 6.