colour bar

Short definition from Wiktionary

Noun

1. 1
   The segregation of people of different colour or race, especially any barrier to black people participating in activities with white people.
2. 2
   A pattern of varying tonal density that enables visual and numeric comparisons to be made across multiple printed sheets or pages.
3. 3
   A layer of good jewellery-quality opal within a rough opal.