Patterncolor

What is meant by Pattern here

Wildtype chicken can be viewed as groundcolored with areas of darker coloration, either entire body parts and feathers, or areas on individual feathers. This dark/black coloration consists of eumelanin and is often referred to in this wiki as 'pattern'.

Eumelanin

Eumelanin, often abbreviated to melanin, is the more photoprotective pigment, appearing black or in a lesser quality brown (fancy name: 'chocolate'). When present in low amount or clumped the pigment appears grey (fancy name: 'blue').

Eumelanosomes

Eumelanin is packed in so called eumelanosomes. These are lysosome like intracellular vessicles that have a fibrillar matrix to which the pigment adheres. A building block of this fibrillar matrix is the PMel17 protein. Mutations of its gene can reduce eumelanin expression up to 100%. Dominant White, symbol I, is the fancy name for its most powerful mutation known to date.