color is a built-in turtle characteristic that represents the color of each turtle in a NetLogo model. Every time we create new turtles, each one is assigned a random color. We can use the set primitive and the color primitive together to change a turtle's color as follows: set color red.

ask turtles [
    ifelse size > 2 [
        set color red
    ][
        set color green
    ]
]

color is also a reporter; we can use it to access the color of a turtle as follows:

ask turtles with [color = red][
    set size 5
]

Things to keep in mind when using color:

• NetLogo uses a custom numbering scheme to represent colors. You can access this color numbering scheme by clicking the Tools menu and choosing the Color Swatches option.
• As you noticed in the examples above, you can use the names of the common colors directly in your code. These names are: red,green,blue,brown,black,pink,white,violet,magenta,cyan, and gray.
• You should write the name of a color directly, without any quotations around the color name ("").
• As NetLogo uses a simple numbering scheme for colors, you can actually treat color names like numerical values. This allows us to manipulate the lightness of colors with simple math. For example, green + 2is will result in a lighter green color, while green - 2 will result in a darker green. You can also use non-integer numbers to achieve an even more precise color such as red + 0.25or blue - 1.85.

In the model example below, we have a white sheep and some plants. We use the color primitive to make some of the plants green and some yellow. We use the ask turtles with [color = white] to just ask the sheep to move, while the plants remain stationary. Our sheep moves around randomly. When there is a yellow plant on the same patch, our sheep just ignores it. However, if there is a green plant on the same patch, our sheep eats the green plant.