Via Lauren Kitchens:
Storage: The day after you make the buttercream you will need to whip at medium speed for 5 minutes. Store at room temp (68-74 degrees F) for up to 2 weeks. (Yes, you heard her correctly). Store in the fridge for up to 2 months. Store in freezer for up to 6 months.
Color: Use gel paste icing to color to tint. If you need a deep color, heat a small portion of the tinted butter cream in the microwave at 50% power, 5 seconds at a time to deepen the color. Do not melt. Add it to your buttecream and whip.
Egg Whites: Purchase the pasteurized egg whites i the grocery store next to the eggs. You can use fresh egg whites but it will decrease the shelf life of the buttercream unless you pasteurise them yourself. I do not recommend using meringue powder as a replacement.