Dye is transparent, so the original color will always affect the resulting color. If you use a perfectly balanced grey dye, there will be a very slight amount of purple in the results, from the original pale lilac. However, since the original is pale, its contribution may be so small as to be difficult to detect. You may need to use a bit of purple dye, in addition to grey.

Note that you can dye the pants only if they are washable. In addition, the stitching is probably done with polyester thread, which cannot be dyed. It will remain the original color. In some cases, the contrasting stitching looks good, but in others it looks bizarre.

Silk can be dyed with any of the various types of acid dye, such as One Shot Dyes, Washfast Acid dyes, and Kiton Acid dyes, or with fiber reactive dye such as Procion MX or Sabracron F dye. See the acid dyes and the instructions at http://www.prochemical.com/directions.htm , to decide which dye you wish to use. The soda ash recipe that is easiest to use with silk, because no heating is required, may soften some weaves of silk and make it less shiny. Acid dyes require heating, but are best if retaining the shine is very important to you. It's a matter of taste. The color chips for the fiber reactive dyes are based on cotton, however, and the colors of the mixed shades, such as grey, may turn out to be quite different on silk. Acid dye on silk is more predictable, as far as getting the color you plan on getting.

(Do not use acid dye on cotton. For cotton, it is important to use fiber reactive dye.)