I would like to make poinsetta cupcakes but need advice on how to get truly red buttercream, and not sickly rose pink! Thanks!

Just keep adding red. I usually add different reds (red-red, christmas red, no taste red, etc. depending on the color I want). It will darken after it sits. I would get a color close to what I want & let it sit for a few hours as it will deepen. The only way is to just keep adding color. In the beginning I was afraid to use "too much" but I have often used a whole bottle of color to achieve the hue I needed. HTH