More technically known as SMPTE color bars (after the Society of Motion Picture and Television Engineers), they were test patterns used to calibrate TVs.

Learn how to tweak your desktop or laptop display using free test patterns or the built-in utility in Windows or Mac OS X.

What looks like random blocks of color to the untrained eye is actually a carefully laid-out test pattern. For example, the colors yellow, cyan, green, magenta, red, blue actually goes from ...

The test patterns can be downloaded from the AVFoundation page on Apple's website under Related Resources > Color Test Patterns, with complete instructions available in Apple's support document.