Mixing Colors Digitally - Searching for Greens
The first post in this series, “Mixing Colors Digitally—Introduction,” explained that mixing colors with physical pigments basically involves two processes: a physical process that’s subtractive, and a psychological process of viewers additively averaging the colors they see. Painting software doesn’t try to mimic either process; instead, it mixes colors by weight averaging their red, green, and blue components.
In this post I’ll share two color mixing experiments—to mix green from certain blues and yellows—that clarify how these disparate mixing processes produce different mixed colors.
Displaying Mixed Colors Graphically
First, let me explain how I display the results of the color mixture experiments. The original colors and mixed colors will be converted to HSV (hue, saturation, value) color space. Then I'll plot each color as a dot on a radial graph—the direction of the dot from the center indicating the color’s hue, the distance from the center showing its saturation, and the size of the dot showing its value.
For instance, the first experiment starts with colors that resemble Daniel Smith watercolors Hansa Yellow Medium and Ultramarine Blue. The yellow color is (253, 236, 35) or #FDEC23; in HSV color space, its hue angle is 55, saturation is 220, and value is 253. The blue color is (28, 74, 209) or #1C4AD1; its HSV hue angle is 224, saturation is 221, and value is 209. Here’s how I display those colors graphically:
What does the graph show? First, positions of the dots show these colors have nearly complementary hues and should mix to gray (at the center of the graph). Second, the dots being close to the edge shows the colors are very saturated. Finally, the slightly larger size of the yellow dot shows that color is relatively lighter than the blue.
How does this compare to mixtures created by painting software? The next graph plots some mixtures (with 9:1, 8:2, 7:3, and so on, ratios of yellow to blue) that can be created digitally:
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| Dots for colors resembling Hansa Yellow Medium and Ultramarine Blue |
Experiment 1: Hansa Yellow Medium and Ultramarine Blue
The Australian watercolorist Jane Blundell is an avid explorer of color mixing. On the Daniel Smith blog she searches for green mixtures from some of the company’s yellow and blue watercolors. Here are the colors she mixes from the Hansa Yellow Medium and Ultramarine Blue pigments: |
| Pigment Mixing Path from Hansa Yellow Medium to Ultramarine Blue |
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| Software Mixing Path from Hansa Yellow Medium to Ultramarine Blue |
Notice, first, that the software’s weighted-averaging mixing path is more uniform than Blundell’s mixing path. The software path is almost a straight line with the mixed colors bunched toward the original colors at either end of the path. (Admittedly, the software path is not perfectly straight. It would be straight if the RGB / HSV color space were perceptually uniform in this region, like it is along paths leading from monitor red, green, and blue to their complementary colors.) Second, the software path passes closer to the middle of the circle, which is achromatic gray: in other words, its mixed colors are less saturated than those in Blundell’s path. This explains why we cannot digitally mix nice greens from this yellow and blue. There is a green (hue 120) along the software path, but it’s so unsaturated that it looks gray. On the pigment path, by contrast, the true greens (hue 120) are more saturated, and some of the saturated, dark yellows (in the hue 60-90 range) will look green to most observers.
The differences between these two paths—the ability to mix from the pigments more saturated colors, including some muted greens—can be explained only in part by the subtractive factor of color-mixing with pigments. We can see this by comparing the paths above to this purely subtractive mixing path created in software:
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| Subtractive Mixing Path from Hansa Yellow Medium to Ultramarine Blue |
(Technically, the software can’t mix colors subtractively on a single layer. Rather, it blends them when you paint the colors on separate layers and set the top layer to Multiply blend mode. This emulates shining full-spectrum light through two colored gels. You can vary the blend by choosing which color is on the top layer in Multiply mode and adjusting the opacity of that layer.)
The mixed colors on the purely subtractive mixing path are quite saturated (the path swings wide toward the edge of the circle), but very dark (the dots are small). Thus, the subtractive process in pigment mixing can explain why pigment-mixed colors are saturated, but not why they are light in value. I suspect these pigments mix to relatively light colors because we additive-average the various colors reflected from them, a process that tends to produce lighter neutral tones.
Experiment 2: Hansa Yellow Medium and Cerulean Chromium Blue
Because digital color mixing paths are (almost) straight, we expect more saturated mixed colors when the original colors are closer on the hue circle. So, we might get a more saturated green by switching to a blue that is closer to yellow. In this next experiment, I replace the Ultramarine Blue with a color that resembles Daniel Smith’s Cerulean Chromium Blue watercolor. This blue is (50, 132, 192) or #3284C0; its hue angle is 205, saturation is 189, and value is 192. Compared to the Ultramarine Blue above, the Cerulean Chromium hue is almost 20 degrees closer to the Hansa Yellow Medium hue.
Here are the colors that Jane Blundell mixes from these pigments:
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| Pigment Mixing Path from Hansa Yellow Medium to Cerulean Chromium |
Once again, the pigment mixing path is a ragged. The relatively dark, saturated outliers in the hue 90 to 120 range are the mixed greens so valued from these two pigments. Also, some lovely green-yellow mixtures are light and saturated.
The software mixing path is more predictable. It lacks vibrant dark greens, but it includes some lighter, less saturated green-yellow colors:
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| Software Mixing Path from Hansa Yellow Medium to Cerulean Chromium |
The purely subtractive mixing path, as we’d expect, swings wide to produce very saturated, but darker yellow-green colors:
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| Subtractive Mixing Path from Hansa Yellow Medium to Cerulean Chromium |
Notice the large gap between the 5:5 and 4:6 yellow to blue mixtures, just where a true green (hue 120) would be. Could this help explain why Blundell’s pigment mixtures are relatively unsaturated in the range from hue 110 to hue 150?
Conclusions
Mixing colors with physical pigments is complex and quirky; it’s largely dominated by a subtractive physical process but also involves a psychological process of viewers additively averaging the colors they see. Mixing colors digitally, on the other hand, is a simple process of the software weight-averaging their RGB components.
I ran two experiments that compare the software’s mixing process with Jane Blundell’s attempts to mix green from certain yellow and blue pigments. The experiments suggest:
- Mixing colors digitally has a more predictable mixing path than mixing colors with pigments.
- Digitally mixed colors are less saturated than those mixed from pigments. (Thus, when greens are mixed digitally from yellow and blue, they look almost gray.)
- Colors blended subtractively in software are more saturated and dark than those mixed from pigments.
- To mix a reasonably saturated green color digitally, the mixing colors must be near the target color (within 30 radial degrees) on the hue circle.
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Thanks for reading!
I hope that you enjoyed this post and that it inspires you to enjoy digital painting. If you find this post helpful, please share it with your friends. And please send me your insights on digital painting and suggestions for Digital Paint Spot.
Bob Kruschwitz
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