Color matching between the lycra and silk isn't as easy as you might think. There's no simple formula, and it just takes time and patience.
There are a 2-3 colors where I can purchase silk to match the lycra and it's a decent color match. That's a bonus when that happens, because it helps keep the time, and therefore price, down. This Romeo and Juliet dress is a great example of a "stock" silk matching up with a "stock" lycra.
Dresses are one-of-kind, and dyeing is done in small batches. The trickiest part of color matching is combing the dyes as a good starting point, and then correcting during the dyes process (over near boiling water)
Here's the starting point for deep coral - 4 colors. I then run a series of tests as I get close to the hue. Once the color is close, I often have to correct for the saturation of the color. I know that when the color matches while the fabric is wet, once the fabric dries it will be too light. I also have to be careful that the color isn't made too deep/dark.