Simple Coloring: two colors, one connected argument

Track one digit

Join conjugate pairs: two positions in a unit with exactly one true. Give their endpoints opposite colors and continue through connected links. A mere no-repeat link is insufficient because both endpoints could be false.

HoDoKu — Coloring · SudokuWiki — Simple Colouring

Our color trap for 6

Let the only 6 positions in column 2 be B2/H2, in row H be H2/H8, and in column 8 be H8/C8, with no placed 6 in these units. The chain B2–H2–H8–C8 can be colored red–blue–red–blue.

B7 sees red B2 through row B and blue C8 through the top-right box. Whichever color is true, a 6 blocks B7. Remove 6 there without deciding which color wins throughout the component.

A same-color conflict is a different rule

If two cells of one color in the same connected component see each other, that color cannot be true. Eliminate it from that component. Unlike a color trap, this resolves a color. Identically named colors in unrelated components have no such shared meaning.

Every color needs a reason

Adding a third 6 at E2 breaks the B2–H2 conjugate pair. Recheck the colored connections rather than keeping the old picture. Every link should lead back to a complete, verified pair of positions.

In Classic 9×9, use Scan to scan the current entries, inspect justified eliminations and open this lesson. Chain and ALS searches are bounded; UR Type 1 and BUG+1 require verified uniqueness. Reading this lesson never changes your saved game.

Try before looking

References and further reading

Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.

• HoDoKu — Coloring
• SudokuWiki — Simple Colouring