There are two things that can turn yellow, the preserved milk or the resin. Every resin will turn yellow at some point, because of oxydation. The milk can also turn yellow if not well preserved. The recipe on Etsy looks very sound and rational, as the ingredients will all help to prevent everything from degrading. However, I suspect the result will be whity-white, not really the color of human milk...
All of the ingredients prevent microorganisms from deteriorating the milk. This is something I think you can achieve with heating and dehydrating well. I also skim my samples, because fat doesn't seem to go well with resin. What I made two years ago was a pasteurized milk torouhgly dehydrated and it preserved well for two years... However, it wasn't kept in the sun. And I didn't make a jewel back then.
No matter what you try, this recipe or another, you can inspire yourself from tried and true manners of preserving food stuff: desinfecting everything, heating, dehydrating, removing air (sous vide), using chemicals to make the food inhospitable for microorganims, like using salt. One could even think of radiation. But since you are not going to eat it, you can also inspire yourself from the cosmetic industry. It makes a very large playground. :)
My personal goal is to try to stay very close to the milk itself. I do not like how white a lot of milk jewel turns out. I fell what I am seeing is more the colour of the preserving powder, not the actual milk. So I'm continuing to experiment.
