Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
Is Rainbow Sudoku just sudoku in colours?
Rainbow Sudoku is classic sudoku with nine colours in place of the digits: red, orange, yellow, green, blue, purple, pink, teal and gold. Each goes once in every row, column and box, and each colour has a number, from red at 1 to gold at 9. Views, sizes and levels follow the same pattern in all 21 of our picture games, described on Themed Sudoku.
It is also the one game of the 21 with no pictures at all. The other twenty deal emoji, which each phone and computer draws in its own style; these tiles are plain discs the page paints itself, in nine fixed colours, the same on any screen. With no dog, rocket or pumpkin to name, the grid reads as a pattern rather than a scene. It is the quietest board we make: a slow 9×9 here feels closer to a mosaic than a puzzle.
The colours join in rainbow order as the board grows: red, orange, yellow and green on the 4×4, blue and purple added on the 6×6, pink, teal and gold on the 9×9. A young child new to the rules should start on the 4×4, with Sudoku for Kids as the teaching guide.
Why nine colours, when a rainbow has seven?
Because seven was never a count. A rainbow is one continuous run of hues, “far more than the human eye can distinguish”, says the Met Office. Red, orange, yellow, green, blue, indigo and violet are Isaac Newton’s names, seven to mirror the steps of a musical octave (Colour Literacy Project). Our 6×6 is his list without indigo, which modern lists often fold into blue and violet, and with purple for violet.
Two of the 9×9’s three extra colours were in the rainbow all along, unnamed. Measured as hue, the angle round a colour wheel, our teal (187°) sits between green (123°) and blue (208°), and our gold (37°) between orange (34°) and yellow (49°). Pink (340°) is the one no rainbow supplies: it lies where the wheel runs from purple back to red, and a rainbow stops at violet.
Which colours look alike?
Gold and orange. Gold sits less than four degrees of hue from orange, just lighter and yellower. On a phone-width screen in Chrome on Windows, light or dark, the two can be told apart side by side, but met one at a time across a 9×9, where each disc is about 28 pixels wide, either passes for the other. Gold is the last colour in the set, so the pair only meets on 9×9. No other pair looked alike to us.
Tap a gold to see every gold: selecting a square that holds a colour lights up every copy of it. Or switch to Both, where orange carries a 2 and gold a 9.
We measured the difference between all 36 pairs, on the page’s own colour values, with CIEDE2000, the standard formula for how different two colours look. Orange and gold are 9 apart; the next pairs, yellow and gold (16) and red and pink (18), nearly twice that. Every pair that meets on a 4×4 or 6×6 board is at least 25 apart.
Can you play Rainbow Sudoku if you are colour-blind?
Yes, with the numbers on. Red–green colour blindness, the commonest kind, affects about 1 in 12 men and 1 in 200 women of Northern European descent, and fewer in most other populations (MedlinePlus Genetics, US National Library of Medicine). Red and green share every board here, from the 4×4 up.
We ran the nine colours through a standard simulation of colour blindness (Machado, Oliveira and Fernandes, 2009). In the strongest red–green forms, all four 4×4 colours become shades of one olive yellow, told apart only by lightness: with deuteranopia red and green come out almost identical, with protanopia orange and green. Blue and purple draw close in both, and gold all but merges with orange. Milder forms blur the same pairs less. For a child with red–green colour blindness, the smallest board is the hardest to read, not the easiest.
Choose Both under Display: every disc gets its number on a badge, and pencil marks are written as digits. Numbers takes the colours off the board altogether. In every view the key above the board pairs each colour with its number, and on a computer the number keys place colours, so nobody has to pick a disc by its shade.
Where does the green go? A hidden single in a row
Here is the red–green pair on a real board: a 4×4 Hard deal from this game, six colours to start and not one of them orange. Row 1 is missing its green. Take its empty squares one at a time and look for a green in the same column or box; any square that has one is out. The last square standing gets the green, whatever else it might hold. That is a hidden single, read along a row.
Neither column 2 nor the top-left box settles it alone; each still has two squares that could take green. The row does. Now take the red at the foot of column 2 for a green: row 1 has nowhere left to put one, which a board with one answer never allows. From here, hidden and naked singles finish the board: we ran the game’s own singles to the last square, and a separate counter found exactly one answer.
Is Rainbow Sudoku the same as Color Sudoku?
Same idea, different game. Color Sudoku is our 9×9 colour board, with its own engine, its own palette and no numbers on screen. Rainbow runs on the engine our picture games share, so it has three sizes, and numbers when you want them. At 9×9 both aim for the same number of colours per level and dig each level for the same solving steps.
| Rainbow Sudoku | Color Sudoku | |
|---|---|---|
| Board sizes | 4×4, 6×6 and 9×9 | 9×9 only |
| Colours, in key order | red, orange, yellow, green, blue, purple, pink, teal, gold | red, blue, green, yellow, purple, orange, pink, cyan, brown |
| Numbers on screen | in the Numbers and Both views, and always on the keypad | none; the keys 1 to 9 follow the button order |
| Opens on | 6×6 Easy, in Both | 9×9 Medium |
| Colours given at 9×9 | Easy 40, Medium 32, Hard 26, Expert 22–24 | Easy 40, Medium 32, Hard 26, Expert 22–25 |
From each game’s own code; Expert ranges from 40 boards each. Six of the nine shades are identical: Color Sudoku’s cyan is our teal.
For a full 9×9 in colour alone, either will do; for a child, or anyone who wants the numbers within reach, play here. The name is used elsewhere too: Rainbow Sudoku, a 2016 book by Philip Riley and Laura Taalman, prints number puzzles where colour adds a rule, such as every red square holding a different number. And for a rainbow in pictures, St Patrick’s Day Sudoku deals one among its first four tiles, with a pot of gold on the 6×6.