Simple Colouring extended across multiple digits via bivalue cells; six rules detect contradictions.
While Simple Colouring works for one digit, 3D Medusa colors multiple digits simultaneously. In bivalue cells, both candidates are assigned opposite colors, linking different digits.
Six rules detect contradictions: 1. Same color, same unit, same digit → Other color is correct 2. Same color, same cell, two different digits → Other color is correct 3. Uncolored cell sees both colors → Eliminate that candidate 4. Bivalue cell with one colored candidate → Eliminate the uncolored one 5. All candidates in a cell are the same color → Other color is correct 6. Digit in a unit only in one color → Other color is correct
69 | 2 | 69 | 8 | 5 | 7 | 3 | 4 | 1 |
| 1 | 3 | 4 | 29 | 6 | 129 | 5 | 789 | 789 |
| 5 | 7 | 8 | 1 | 3 | 4 | 2 | 6 | 9 |
| 2 | 5 | 1 | 7 | 4 | 3 | 9 | 8 | 6 |
169 | 689 | 3 | 5 | 89 | 1 | 4 | 278 | 278 |
| 4 | 89 | 7 | 6 | 2 | 89 | 1 | 3 | 5 |
| 3 | 1 | 5 | 4 | 7 | 6 | 8 | 29 | 29 |
| 7 | 4 | 2 | 359 | 89 | 589 | 6 | 1 | 3 |
| 8 | 469 | 69 | 239 | 1 | 29 | 7 | 5 | 4 |