Lavender is a recessive gene and the BBS gene is a dominant gene. Lavender requires two copies of the gene to express. A single copy will result in black birds that carry a Lavender gene. It's no so much a problem with first generation, but it following generations it will be impossible to tell which birds have which genes. The dominant Blue/Splash will override the Lavender color. You could end up with birds that are both Blue or Splash and Lavender. But you won't be able to tell that they are Lavender. You won't be able to tell which chicks are Lavender and which are Splash.
Best just to keep the colors separate.
Yeah, what she said. Haha