The best replacement for yolks is egg whites.

The best alternatives to yolks are rated as following:
★★★★★  egg whites
★★★★  whole eggs
★★★★  whites
★★★  yokes
  whole egg

Your alternative for yolks is not listed? Suggest!
Speaking about yolks, one probably shouldn't forget about whole eggs.

Other search terms related to yolks and their ratings are:
★★★★★ whole eggs
You have in mind a companion for yolks and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of yolks in all years. Click on graph for more.
yolks
egg whites
whole eggs