I have a three tier cake on Saturday and I purchased Yellow Satin Ice but it's way too bright for what I am looking to achieve. Can I just mix it with white satin ice to tone it down or should I just buy more white and color it myself?

mix with the white if you have it.

If the brightness is the issue, add brown. It will give it an earthy mustard tone. Adding white will make the cake lighter.

add white