An adult Cesky Terrier can be a light coffee color, or any shade of gray ranging from charcoal to platinum, but all Cesky puppies are born black or chocolate brown.
Because the Cesky was created from both the Sealyham Terrier and the Scottish Terrier, it is allowed to have up to 20% white markings, but more than that is a disqualification in a show ring. The Cesky also inherited from the Sealyham a gene for lightening the pigment, but if the recessive is present instead, pigmentation will stay dark. The brown marker in this breed is recessive (meaning that both parents must be B/b to produce brown puppies).
Genetic “nerds” may find this paper on color genetics in the breed written by Glenn Dorsey of interest. His opening paragraph alone is fascinating as it mentions that of the approximately fourteen genetic markers that have been hypothesized for determining coat color, roughly one half of them have been identified, located on the chromosome, and characterized, and that commercially available tests can be performed on dogs to identify the specific markers. Unimaginable twenty years ago!