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.
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.