One of the toughest aspects of creating digital art is grasping how computers produce different colors. Mixing red, green, and blue light is quite different than mixing colors of paint. However, choosing colors via RGB values is fundamental to working digitally.
This exercise will help you intuitively feel how red, green, and blue light combine to create the entire gamut of visible colors. We will also explore the opposite of additive RGB color mixing: subtractive CMY (cyan, magenta, yellow) color synthesis.
Tools and Setup
I will be demonstrating in Photoshop CS4 for this tutorial. Any version of Photoshop that supports painting in Multiply and Linear Dodge modes should work fine though. It’s also important to use a pressure sensitive pen tablet, because we will be painting with only three colors, with opacity controlled by the stylus.
Choose brushes that vary their opacity with pen pressure. There are several default brushes in Photoshop that have this characteristic. Also set the eraser of your stylus to use these brushes. You should be able to make marks like this:
Once you have the brushes set up, we can start experimenting with mixing color.
RGB Color Mixing
First let’s practice by creating a color wheel. Create a black layer, and lock its transparency by clicking the transparency lock icon in the layers palette. Then set the background color to black. Set the brush tool to Linear Dodge (Add) mode and the eraser tool to Normal mode.
Using only pure red, green, and blue, practice creating colors by sketching a color wheel. Red and green make yellow, red and blue make magenta, and blue and green make cyan. All three colors combine to make white. You can create a grayscale ramp by applying equal parts of each color in a gradation.
Try making something like this:
Try to get a good sense of how the three primary colors interact before you move on.
RGB Color Mixing from a photo
Once you’ve got the feel of creating colors by mixing red, green, and blue light, you can try something much more challenging: replicating a photograph.
First, choose a reference photo to work from. I chose this photo of some peppers because it had some bright, bold, colors, but also some subtle muted tones. It doesn’t matter too much what photo you choose—anything will be a challenge!
Put the photo on its own layer. Then make a new layer over it and trace the photo with a fine, white line. (Tracing is optional, but this is a color exercise, not a drawing exercise, so we don’t want to focus too much on scale and proportion.) In between the trace and the photo, add a layer of black, and you should get something like this:
Make sure your brush tool is still on Linear Dodge (Add) mode and set to work painting on the black layer. You are using only pure red, green, and blue. I started by blocking in the main colors like this:
After I had the main shapes defined, I started layering colors to create secondary colors. For example, I used red and a little green to make orange. I also started blocking in the background.
I’m continuing to layer colors a little bit at a time. I added some highlights by mixing all three colors, and continued building up the background tones. If an area ever gets too bright, I use the black eraser to push it back down.
As the colors approach a greater degree of accuracy, most areas of the painting now have some amount of each color component, since there are very few pure colors in photographic images.
Finally, I used the blend tool to solidify the forms and remove some of the anomalies produced by layering colors:
By this point you should have a good feel of how red, green, and blue light combine to create different colors. Now let’s do the same exercise in reverse!
CMY Color Wheel
Monitors add red, green, and blue light to black to create colors. However, printed media, such as magazines, subtract from white using cyan, magenta, and yellow. To get used to how this works, let’s create another color wheel, this time using subtractive color.
Create a white layer and lock its transparency. Set the background color to white and set the brush tool to Multiply mode. Using only cyan, magenta, and yellow, try to create a color wheel similar to this:
In subtractive color, magenta and cyan make blue, yellow and magenta make red, cyan and yellow make green, and all three together make black.
CMY from a Photo
Now let’s try using CMY color with a photo. Create a white layer with locked transparency and invert the trace of your photo so that the outlines are black.
Using only pure cyan, magenta, and yellow, start to block in the shapes from your photo.
It won’t look a whole lot like the photo when you start… Don’t worry, you’ll get there! Keep on layering colors:
Here I’ve finally started combining mixes of all three colors to achieve darker tones:
At this stage I’m mostly working on the background in order to darken it to an almost neutral gray:
I continue lifting colors with the white eraser and adding them back in with the brush until I’m happy with the accuracy of the colors:
As a final step, I blend some of the forms for continuity and to get rid of the visual noise created by layering:
I hope this exercise helped you get a better sense of how colors mix together both on the computer screen and in printed form. If you think about it, the entire job of the artist is to determine the color of each pixel in an image, so color mixing is absolutely fundamental to what we do! You certainly won’t want to create every image using the method I’ve shown here, but I hope it has helped you get a better sense of how the computer thinks about color.