Simple Coloring uses conjugate pairs (two cells in a unit that are the only places for a digit) to build alternating chains. By coloring these chains with two colors, if a cell sees two cells of the same color, that color is invalid. If a cell sees both colors, the digit can be eliminated from that cell.
Find a digit that has conjugate pairs (exactly two positions in a unit). Link conjugate pairs into a chain and color them alternately. Look for contradictions or eliminations.
In a conjugate pair, exactly one cell has the digit. Coloring alternates: if cell A is colored blue, its conjugate partner is green, and so on. If two cells of the same color see each other, that color is impossible. Any uncolored cell seeing both colors cannot hold the digit.
Simple Coloring builds a chain of conjugate pairs for a single digit. Let's trace digit 1. First, find conjugate pairs — units where 1 appears in exactly two cells.