Mixing Colors Using Fractions

I am trying to create a color mixer that mixes multiple colors together, meaning 2 or more. I'd like the end result to look something like trycolors.com where you can choose how much of a certain color you'd like to put in. For example, let's mix a color that is one part yellow, three parts red, and four parts blue. This would make the resultant color a mix of 12% yellow, 38% red, and 50% blue. How would I go about doing this? I know I probably need to use the Color.Lerp() method, but how would I implement different colors and different divisions?