Medium technique
Last Possible Number: The Only Color That Fits
Also called the last possible number, or a naked single spotted without notes. In Bubble Sudoku it is an Obvious Single: Last Possible.
Every cell sits in a row, a column, and a block. Count the colors in all three. When you count eight different colors, the ninth is the answer.
1
Count the colors around one cell
BEFORE
Look at this empty cell. Its column has Red, Orange, and Yellow. Its row has Light Green, Dark Green, and Turquoise. Its block adds Dark Blue and Light Purple. That is eight different colors.
2
Place the last possible color
AFTER
A cell cannot repeat a color from its row, column, or block. Eight colors are ruled out, so only Dark Purple is left. Place it.
Cells being readKey cellColor tints match the color being followed
Key Insight
Count the row, column, and block together. This works best where all three are busy. When your count reaches eight, place the ninth color.
Related Techniques
Full HouseAlso counts to eight, but inside one row, column, or block at a time.Obvious Single: With NotesThe same idea read from notes. A cell with one note left is a cell where eight colors are already ruled out.If you’re stuck on a puzzle, see what to try next, in order.