The color  appearance of an object depends upon the colors (wavelengths) of light that are reflected by the object to the observer's eyes. The object can interact with the incident light to absorb one or more colors of light. This is called color subtraction. The colors that are not absorbed or subtracted from the incident light are reflected to the observer's eyes.
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The Fundamentals
This Concept Builder will demand that you understand both color addition and color subtraction. The graphic at the right is a great starting point. It shows the three primary colors of light - red, green, and blue. It also shows the three secondary colors of light - cyan, magenta, and yellow. It is important that you understand that a secondary color of light can be thought of as being equivalent to a mixture of two primary light colors. In other words ...

• Magenta light = Red light + Blue light
• Yellow light = Red light + Green light
• Cyan light = Blue light + Green light

This is called color addition. 

Now as you approach a question like this one, you think in terms of primary light colors. For instance, you think of magenta light as being red + blue light (or cyan as being blue + green light, etc.).


The Incident-Absorbed-Reflected Model
A useful model for analyzing color questions is what we call the incident-absorbed-reflected model. The model suggests that a successful analysis includes three steps ... with each step involving the answering of a question. Those steps or questions are ...

1. What primary color(s) of light are incident upon the shirt?
2. Of the primary light colors that are incident, which color(s) is/are absorbed by the shirt?
3. Of the primary light colors that are incident, which color(s) is/are reflected by the shirt?

The absorbed light color(s) is/are subtracted from the incident light colors. This is called color subtraction. Once the subtraction process is complete, the result is the reflected light color. It is the reflected light color that reaches the observer's eye and determines the color appearance of the shirt.


Applying the Principles
In the example question (see About This  Question), there are two diagrams that provide sufficient information for deciding how the shirt interacts with the three primary colors of light. In the first diagram, yellow light shines on the shirt and it looks red. Yellow light is equivalent to red and green light. The fact that the shirt looks red means that the shirt absorbs the green light and reflects the red light. In the second diagram, blue light shines on the shirt and it looks blue. The fact that the shirt looks blue means that the shirt reflects the blue light. So from these two diagrams you can reason that the shirt is an absorber of green light and a reflector of red and blue light.

Now the third diagram shows white light shining on the shirt and you must predict its color appearance. White light includes all three primary colors of light. So the red, green, and blue are shining on a shirt that absorbs green light and reflects red and blue light. So the red and blue will be reflected to an observer's eye and the shirt will appear magenta (the equivalent of red and blue light).

Regardless of which one of the two forms of this question you receive, the thinking will be the same. Interpret everything in terms of primary light colors, determine which primary light colors the shirt absorbs and/or reflects from the first two diagrams, and then apply the information to the third diagram to determine its color appearance.