How many colours should a color by number picture have?
More colours sounds like more picture. It is the one intuition in this area that is reliably wrong, and the reason is a pair of curves moving in opposite directions.
More colours sounds like more picture, and it is the one intuition in this area that is reliably wrong.
Two curves moving in opposite directions
Think about what you actually do with a colour: select it, find every region of it, fill them, move on. Two quantities decide whether that is pleasant.
The first is how hard the regions are to find. Hold the number of regions in a picture fixed and add colours, and each colour gets fewer regions spread over the same area. Rarer targets in the same space mean more scanning for each one you find. The slow part of any picture is the last few regions of a colour, and more colours simply means more last-few.
The second is how much of the canvas each colour occupies. Part of what makes working one colour at a time satisfying is that completing a colour visibly changes the picture. Divide the same area among more colours and each one covers less of it, until finishing a colour is an event you have to look for.
There is a third effect, quieter than the other two. As a palette grows, its members necessarily move closer together. Twelve colours can be twelve colours anyone can tell apart. Forty cannot — several will be shades a phone screen renders as very nearly the same thing, and now identifying which region belongs to which becomes genuinely difficult and the numbers stop being a check and start being load-bearing.
Against all of that, what does adding colours buy? Smoother tonal transitions. That is a real benefit, and it is bounded: past a certain point the extra steps sit inside the noise of the display, the ambient light and your own adaptation, and nobody can see them.
Why twelve works for pattern art
Numbrush carries twelve colours in every picture. That is a design judgement rather than a derived constant, but the reasoning behind it is specific.
On shading: a band shaded from light to dark reads as smooth with a surprisingly small number of steps, provided the steps sit on a deliberate ramp rather than being sampled out of a photograph. Four or five tones of one hue, with clean boundaries between them, already reads as a gradient at arm’s length.
On budgeting: twelve is enough for two or three hue families with three or four tones each, plus a couple of accent colours and a ground. That happens to be the structure most ornamental pattern work needs, which is why the number is not arbitrary even though it is not forced.
On presence: with a few hundred regions and twelve colours, each colour lands in tens of places. That is enough for completing one to change the picture, and few enough that the stragglers are still findable — particularly since selecting a colour highlights every unfilled region of it across the canvas at once, which is a much better search tool than reading digits.
And on session shape: twelve is also twelve natural stopping points. Each colour is a unit of work of roughly the right length, which is a property of the count rather than of any feature.
Curated palettes and clustered ones
There are two ways to arrive at a palette, and they produce noticeably different results.
Clustering
Take an image, group its pixels by colour, and take the centre of each group. This is how a photograph becomes a colouring page, and it is entirely automatic, which is its great advantage — a pipeline that clusters can turn any image into a page.
Its output is an average of what was in the source, and averages are muddy. You tend to get desaturated tones, several near-duplicates wherever the image happened to contain a lot of one thing, and no relationship between the colours as a set, because nothing considered them as a set. The characteristic symptom is two entries you cannot reliably tell apart, which nothing in the process was there to prevent.
Curation
The alternative is that someone chooses the colours to work together: sets the tonal ramps, decides where the accent sits, decides which two hues meet at a boundary. In Numbrush the palettes are hand-curated, and the artwork is drawn to suit the palette rather than the palette being extracted from the artwork.
The difference shows up mostly in the near-misses. A curated palette does not contain two colours that are almost the same, because a person looked. The cost is that curation does not scale the way clustering does, which is exactly why photograph-based apps can carry enormous catalogues and drawn-artwork apps cannot.
There is no correct number, and anybody offering one as a law is overreaching. What is not a judgement call is the direction of the tradeoff: colours are not free, and the price is paid in search time and in how easily the palette can be told apart. Both of those are invisible in a screenshot and obvious twenty minutes into a picture, which is why the count that looks most generous in a store listing is rarely the one that is nicest to use.
Frequently asked questions
Is a colouring picture with more colours better?
Usually not. Adding colours makes each one rarer and harder to find, shrinks how much of the canvas it covers so completing it changes less, and forces the palette entries closer together until some of them are hard to tell apart. What you gain is smoother tonal transitions, and that benefit runs out well before the costs do.
Why twelve colours per picture?
It is enough for two or three hue families with three or four tones each, plus accents and a ground, which is the structure ornamental pattern work needs. It also leaves each colour with tens of regions, so finishing one visibly changes the picture while the last few are still findable.
What is the difference between a curated and a generated palette?
A generated palette is derived by grouping the pixels of a source image and taking the average of each group, which tends to produce muddy tones and near-duplicates. A curated one is chosen as a set by a person, so the ramps are deliberate and no two entries are almost the same. Generation scales to any image; curation does not.
How do you find the last few regions of a colour?
Selecting a colour marks every unfilled region belonging to it across the whole canvas, which is far easier to scan for than a digit. On symmetric artwork those regions also tend to sit at predictable positions, so the last few are rarely a genuine hunt.
Try it on a real canvas
Numbrush is colour by number for adults: mandalas, rose windows and geometric pattern work, twelve colours a picture, a magnifier for the tight spots, and a wrong tap that costs you nothing. Free, offline, no ads and no account.