You mix red and blue first then add in white to you get the color purple you want
Both the people below are right, but they have it backwards. I'm assuming your needing an icing color? Well. Thats what I do. Your best be is to get butter recipe icing, its white in color, and I personally think it tastes the best. Get some food coloring- making sure you have purple. (unless you have both red and blu, but that would be more difficult) add maybe 10 drops of purple to your icing and mix. Keep adding like 2 drops at a time until you get it the shade you need. - you could add white to purple as the people below said. But- since you want lilac- a light shade of purple- it would be easier to do as I instructed.
Pink and blue
I would think if you added a lot of white to either violet or purple
Use purple and keep adding a little bit of white at a time and mix it and just keep adding the white until it gets the shade you need it.