In designing a machine to paint pictures using watercolors, a problem is that the media is usually white and watercolors are transparent making it difficult to paint dark colors.

To solve this, color printers use black ink mixed with the colored inks.

My question: What is a good way to convert a color specified as RGB into RGB plus Black.

  • @user287001 is right, RGB is an additive color model and mixing those colors would limit the range. CMYK is made specifically for printing with halftone screening and won't give good results with liquid inks. How is your printing creating tints of the inks? I don't suppose you are using screening. Are you just watering down the inks and apply them one at the time or are you actually mixing the ink before applying it? – Wolff Mar 31 '18 at 9:34

This seems (see NOTE1) quite a bad misconception of how watecolors work on paper. Opposite than RGB colors on the computer screen, watercolors do not add anything, they take off parts from what the paper reflects. Onscreen, if you add the amount of R,G or B, you will get more luminant color , finally the max. white of that screen when R=G=B=255. On paper adding more color makes the result darker.

General terms: RGB is additive, watercolors are subtractive

Of course there exist pigment color materials which are intended to be as opaque as possible. They are essentially paints. They do not need reflective white paper. Mixing them is even more complex than with subtractive colors and beyond the scope of this answer. (to stay in truth, watercolors also act like opaque paints when there's enough thick layer of color. Only thin layers can be considered subtractive )

If you planned to mix red, green and blue watercolors and then add black, if needed, you will get only very muddy and narrow subset of the whole range of the colors, which is possible with watercolors. Our eye physics demand to mix cyan, magenta and yellow (and black, if needed) to cover any useful part of visible color range with subtractive mixing of 3 colors and black.

Normal CMYK color printing probably is already as good as it can be. There's also left invisibly small white areas between the colored dots because substantially more colors are available this way. It compensates the non-ideality of the inks. Better results need more different inks.

I have seen a watercolor artist to use at least fifteen different colors in one work and mostly as mixed. I asked "why do you need that many, why do you not simply use CMYK; only four bottles and water?" Fortunately I had fast feet and I survived. Now I know, that CMYK wouldn't produce wide enough color selection as watercolors. Obviously print colors are different and the microscopic white areas are essential.

My suggestions:

1) revise yor knowlwdge of color theory

2) make tests, create a mixing chart of few basic watercolors - say two blues, two reds, two yellows, black and of course, the water. To get their rgb equivalents take a photo (you must have consistent white light for photos and you must be able to get the colors right - simply=pro equipment and skills!) and take RGB samples in Photoshop or GIMP.

3) You must develop some custom software algorithm which tries to convert RGB colors to the ones which are seen to be available. Again something absolutely non-trivial. But not impossible, if you can develop the software for the painting robot.

Color mixing is non-linear and heavily dependent on used color materials. You will get surprised how small amount of blue makes yellow to green. See this link. It's, of course not for your colors, but can be useful to learn something:


Color theory was in the past based only for visual experiments due the lack of math-physical understanding of light. See this article of "seen color":


ADD due a comment: Red, yellow and blue (=RYB) were the old basic colors of painters. Thanks to user @Wolff to mention it.

NOTE1: This all is based on guessing your intentions. Let me know, if I guessed wrong so I can apologize and delete this.


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  • Good answer. It's also worth mentioning the RYB color model, which was traditionally used for mixing wet paints. – Wolff Mar 31 '18 at 9:20

That's not going to work.

Watercolour paints are not RGB colours plus black.

As others have noted, paint = subtractive colour, and RGB = additive colour.

A typical watercolourist's basic paint set contains around 12 colours at minimun, all of which can all be mixed with each other.

One example, a Windsor and Newton Cotman Watercolour Paints 12 half-pan set contains the following colours: lemon yellow, cadmium yellow, cadmium red pale hue, crimson alizarin, ultramarine, intense blue, emerald green, sap green, yellow ochre, burnt sienna, burnt umber, china white.

So, to describe these colours digitally, you'd need to invent some colour model which could accommodate at least 12 colours, not forgetting the addition of water which is used to dilute the pigments. Less water = stronger colour. Painting black isn't difficult in watercolours, you simply use less water to get a stronger black.

Also another thing which differs with watercolour in comparison to other mediums such as oils or acrylic, is that watercolour artists paint from light to dark, layering up the colours. Basically they start with light washes of colour (often on wet paper), and layer progressively darker colours on top.

Non-paint techniques for getting white also exist, for example using a wax resist to protect areas of white paper. Also dry scratching and wet scratching can be used.

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RGB is Additive. What does that mean? To say that the primaries are additive means that the image "begins" with no luminance (a dark monitor, say) and becomes lighter as luminance is added. It's additive starting with none.

In effect, black is the palette and you "paint" with light adding luminance to a maximum (white) of each of the three additive primaries.

R, G, B, plus black is… no R, G, or B.

How can an RGB image be printed on (white) paper? How can any image be printed? This is a situation that we face every day in the graphic arts. Software engineers came up with "colour break/colour separation" algorithms that convert one colour system to another to accommodate "4 colour process" printing. In the olden days, this was done by a process camera, photographically. (TIP: The negative of R is the "opposite" of R on the colour wheel. Answer: Cyan. The negative of G is the Magenta ink image, and the B component produces the Yellow image. Black is the inverse of the luminance value. Neat, Easy, squeezy.)

4-C printing does a fair job representing the world; but, adding other "spot" colours add to the possible colour gamut available. Adding "orange" and "green" in addition to process Cyan, Magenta, Yellow, and Black makes for much better images.

It's important that the printing (subtractive) inks used are transparent to let the light penetrate the layers of ink going in, bounce off the white background, and pass through the layers of ink a second time on its way to your eye effectively doubling the ink-layer efficiency. No ink at all is the whitest white the subtractive system can create. That it works as well as it does is because of our very forgiving perception and cultural clues creating an optical illusion.

Thus, we need two systems to satisfy our technical need. Additive for luminescent images, and Subtractive for reflective images.

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