What should I add to this to make it more magenta? 

“magenta” is a range of colors — but you need some blue in there —

going forward — make a small amount of icing the color you want in order to test it out — so much easier to handle because after a while the icing starts to get pulled back & forth kwim — but adding some blue icing to a small portion of the icing in the bowl would help you see which way to go ~

Thank you!  It worked!

yay! you're welcome!