You can always add some food coloring to your lather.I'm bored of the lathers of all soaps and creams being white. Why hasn't any manufacturer thought of making a soap that makes a lather of some other color instead? Or is there a technical limitation with soaps that they only produce white suds?
Because lather is mostly air and water (optically neutral with a - very - little soap thrown in). Regardless of the color of the soap, the lather will be lighter. Much. Some soaps, like Stirling's Haverford or B&M's Lavanille, are extremely dark and produce a slightly darker tinted lather. Others like GFT's Violet will be a more distinctive color, as will your brush.
...and what if your $300 white tip badger ends up dyed purple?