Think of the color variations as a spectrum. On one end is the coolest white which is a cool bright white. At the other end of the white spectrum is ivory with its warm slightly yellowish tone. Somewhere in the middle of the spectrum is a color known as diamond white. This color is commonly used to describe white silk gowns since natural silk can never be dyed cool bright white.  Dyeing fabric is more of an art than a science. Like cooking there are always slight differences in dye lots.  Maybe it was more humid or hotter and the fabric absorbed more color.  That is why the only way to ensure an exact match is to order at the same time.

Of course that may not be an option if your dresses are made by different designers.  The best way to ensure a harmonious effect is to keep the dresses on the same side of the spectrum.  Rather than try to match everything perfectly simply determine if the dresses are in the warm family or in the cool side of things. Keep cools with cools and warms with warms.  It’s as simple as that!