The ball traditionally used in Test cricket is red. Its dark color is part of Test cricket’s traditional daytime identity and contrasts with the white clothing worn by players.
A cricket ball has a cork core wound with string and covered in stitched leather. In Test and first-class cricket, the ball is normally red and is expected to remain usable for at least 80 overs before the fielding side can request a replacement under the Laws.
White balls became standard in limited-overs cricket because they are easier to see under floodlights and against darker backgrounds. However, white clothing makes a white ball unsuitable for traditional Test conditions. Pink balls were later developed for day-night Tests, offering better visibility in evening play while retaining many characteristics needed for a long innings.
The color is therefore traditional, not universal. Day-night Tests use pink balls, while one-day internationals and most other limited-overs matches use white balls. Orange balls have been experimented with, but they are not the standard Test choice.