Multi-Coloring (3D Medusa)
Run Simple Coloring on every digit in parallel. Cross-color contradictions across digits unlock eliminations the single-digit version misses.
Advanced sudoku technique
What it is
Multi-Coloring (also called 3D Medusa) runs Simple Coloring across all digits at once. Color each digit's strong-link chain independently using a different color pair (e.g. digit 5 uses blue/green, digit 7 uses red/orange, digit 3 uses purple/cyan). The added power comes from cross-color rules: if a cell has both a colored candidate for digit A AND a colored candidate for digit B, and those two colored candidates together would force a contradiction (e.g. one says the cell IS A, the other says it IS B), then the color that produces the contradiction must be the FALSE set everywhere it appears. That ripples through the network and produces eliminations that no single-digit Simple Coloring would catch.
When to use it
On the very hardest puzzles (evil tier, and the rare expert that resists everything else) where Simple Coloring isn't enough.
Worked example
Digit 5 chain colors R2C3=blue and R2C7=green. Digit 8 chain colors R2C3=red and R5C3=orange. If R2C3 is 5 (blue=true), it can't also be 8 — so red (the 8 candidate at R2C3) is false. That means digit 8's orange chain is the true side, and R5C3 = 8. The deduction couldn't be reached by either digit's chain alone.
Try it
The Sudoku247Online solver walks you through every move of any puzzle one logical step at a time, naming the technique that justifies each placement. Paste a puzzle to see this technique applied in real time.