For watercolor paint swatch database, I used the following 9 tubes of paint from Schmincke brand based on this brand's recommendation for a simple but comprehensive palette:

Lemon Yellow (Pigment: Monoazo PY3)
Cadmium Yellow Light (Pigment: Cadmium zinc sulphide PY35)
Yellow Ochre (Pigment: Hydrated iron oxide PY42)
Cadmium Red Light (Pigment: Cadmium sulphoselenide PR108)
Permanent Carmin (Pigment: Quinacridone red PV19)
Phthalo Green (Pigment: Phthalocyanine green PG7)
Prussian Blue (Pigment: Iron cyan blue PB27)
Ultramarine finest (Pigment: Ultramarine blue PB29)
Ivory Black (Pigment: Carbonized bones of animals PBk9)

Please note that I have no affiliation with this brand. I chose it because of its consistency and pigment quality.

these paints were mixed together to cover the color gamut. Mixtures with 2, 3 and 4-paint ingredients were made. For each mixture, all the ingredients were weighed carefully to achieve the desired proportions. Watercolor White paint was avoided because it makes the colors greyish, chalky and opaque. Water was added to each mixture to lighten up the created hues in different steps. Black paint was used in 2-paint mixtures only so that most dark values were created through mixing complementary colors.

My objective was to make paint mixtures with known concentrations. Therefore, for each mixture, water and the comprising paint ingredients are weighed one by one and mixed inside a glass jar. The main challenge for creating watercolor patches arises from its main ingredient: Water. Water is used to dilute and lighten up the samples, and is added in relatively very large quantity to achieve the light values. Water wets and distorts the paper, runs freely around the swatch space carrying and depositing the pigments here and there, so that the most non-uniform swatch is made. Water is less viscous than gum arabic, the liquid that carries the pigments in a paint tube, therefore, the more water is added to a paint, the bigger the challenge. One might think of avoiding this challenge by using watercolor White paint instead of water to lighten up the paints, however, this will make the colors greyish, chalky and opaque. Transparency of colors is watercolor's most important feature.

To resolve this issue, we pour the sample on a flat silicon brush and sweep it in a continuous forward motion on the paper, so that small amounts of sample liquid flows from the brush and lands on paper and is completely absorbed at that location as the brush moves forward to deposit the rest of the sample liquid. The result is a uniform rectangular patch. The sample liquid amount is constant for all patches but the length of the rectangular patch is longer for samples with higher water proportions, as you can see in the image below. Next, we choose and cut out a small square area within each rectangle patch and paste it alongside other cut out square patches on a new piece of paper to get a nice and clean swatch, as seen in the image below. Each patch of color in the swatch is about 18 x 18 mm (~ 0.7 x 0.7 inch) large.

Here are some quick notes for the attention of the non-color scientist reader. My source for the information is the following book: "Vision and Art, The Biology of Seeing" by Margaret Livingstone, ISBN 978-0-8109-9554-3. It is a very interesting and informative book for artists.

Light, in form of electromagnetic waves, come down from the sun (and other illumination sources) to us. Each electromagnetic wave can be characterized using its "wavelength". (Because we like to assign a "number" to everything so we can work with them easier). Those electromagnetic waves that have a wavelength between 400 nm and 700 nm can be detected by human eyes and can be interpreted in our brains as "colors". Therefore, we refer to the range of 400 nm to 700 nm as the visible spectrum.

For example, an electromagnetic wave with wavelength equal to about 500 nm is usually interpreted in our brain as color green. The reason for this "usually" is that, it depends on the beholder! We are all different in terms of light receptors that we have in our eyes and the way our brain interprets the signals coming from these receptors. That's why a green leaf can look brown to a some color-blind people. To solve this problem, the International Commission on Illumination (CIE) came up with a standard observer. Color scientists use this standard observer in their calculations for consistency. The rendered image below shows the associated color that the standard observer sees by looking at light of different wavelengths in the visible spectrum. We can understand from the image that when we see an object as "green", it means that the object is reflecting mostly the light at wavelengths about 500 nm to 550 nm.



When some rays of light from an illuminant (sun, light bulb, etc) hits an object, the object transmits or absorbs some parts of this light, and reflects the rest of it to our eyes. Therefore, the light that reaches our eyes varies depending on the object that reflects it, but also the type of illuminant. Once again, for consistency reasons, the CIE defines standard illuminants. For example, the illuminant D65, which is used in the calculations in this work, is defined to resemble average daylight. Here is the explanation from CIE (source: http://www.cie.co.at/publ/abst/s005.html):

CIE standard illuminant D65:
(Joint ISO/CIE Standard, ISO 10526:1999/CIE S005/E-1998, CIE Standard Illuminants for Colorimetry)
"This is intended to represent average daylight and has a correlated colour temperature of approximately 6 500 K. CIE standard illuminant D65 should be used in all colorimetric calculations requiring representative daylight... Variations in the relative spectral power distribution of daylight are known to occur as a function of season, time of day, and geographic location. However, CIE standard illuminant D65 should be used pending the availability of additional information on these variations."

For each of the color swatches, I have included in the Swatch Specifications section a plot that shows reflectance factor of the color at different wavelengths. A light spectrum is put as the background of the graph. An example will show you how to interpret the graph: Consider the first color patch. It is made of 240 parts Water + 1 part Lemon Yellow + 1 part Cadmium Yellow Light + 4 parts Permanent Carmin. We can see that the reflectance graph for this color patch has a peak at around 450 nm, which corresponds to the magenta color that comes from the Permanent Carmin paint. Reflectance at wavelengths 600 nm and above is the heighest, corresponding to the constituing Yellow paints and the redness of the Carmin paint. In general the reflectance graph for this patch goes through high values of reflectance factor, because high amount of water has made the color very light by making it very transparent so that the whitness of the paper showes through (Higher reflectance factor means more light reaching to our eyes, which means "lighter").

I used a sphere spectrophotometer, namely the X-Rite Color i7 instrument for the measurements. Full technical specifications and details can be downloaded in the Publications section. In summary, I calculated the X Y Z tristimulus values using the Reflectance factor at 400 to 700 nm, in 10 nm steps, CIE 2nd observer, and CIE D65 illuminant. The colors shown in this website are created by inputting my data into the HTML RGB function. The L* value is used here to represent the lightness (gray vlaue) of each swatch. Appointments to view the actual swtaches can be arranged, please contact me for more information.

You can consult a general view of the palette here. If you click on any of the colors, it will take you to the detailed specifications of that color. Full details can be downloaded in the Publications section.

You can find here the details for each swatch color