A reduction print is printed from one block, no matter how many colors are eventually printed. Each color is printed one at a time, from lightest to darkest. Starting with the lightest color, the artist initially prints the entire print run.
After a color is printed, it is then carved out of the block, and the next darkest color is printed. There is no chance to go back and print the previous color. This process is repeated until the block has been carved away.