Answer: White

For centuries, people have improved the appearance of their clothing through a process known as bluing, but despite what the name might imply, it doesn’t turn fabric blue. The process does add blue dye to the water, but the end goal is not to turn the fabric as blue as the source material, but to make white fabrics appear a crisper, cleaner shade of white.

How does adding small amounts of blue dye to a load of whites produce a crisper looking white fabric? White fabrics yellow over time. Add a little blue dye to the mix and, by the magic of complementary color theory in the subtractive model of color perception, the combined blue and yellow tints make the white fabric appear brighter than in its unaltered state.

Today, laundry detergents may include bluing agents but, unlike their historical counterparts that were actual blue dyes, modern bluing agents are actually fluorescing agents designed to bind to white fabric and give the fabric a brighter white appearance. None the less, if you’re so inclined to experiment with traditional bluing methods to brighten your whites, you can do so—old fashioned laundry bluing can still be found alongside modern detergents and fabric softeners.