How can I make a mayo/ketchup-based sauce come out with a consistent color? I'm trying to make a zesty sauce from a mayo base/ketchup base mixture.  Sometimes color turns out dark orange, sometimes off white, even though I'm using the same recipe.  I'm thinking that how I make the mayo base could be altering the color of the final product.  I'm looking to get a pale orange final color.

I've been making the mayo base using hand wisk, canola oil, white vinegar, lemon juice and egg yolks and like I've said, use the same process each time but can't explain the color difference.  Any suggestions on what I can do to get a more consistent final product color?
I would think the main factor in the colouration, assuming you use the same amount of ketchup each time, is the colour of the egg yolks.

There is a specified range of yolk colours, specified on a yolk colour fan (PDF). The colour varies depending on the amount of caretonoids in the feed the chickens have eaten. Most chicken farms will supplement the feed with supplemental caretonoids in order to avoid pale yolks.

As eggs from the supermarket will likely vary widely in source, and thus feed, supplements and colour, it may be worthwhile to find a local, small-scale farm where you can ask the farmer about feed and supplements. If the chickens are truly free range, they may well eat a lot of insects, which also contributes to a rich orange yolk.