Which of the following is not an option for specifying a color in CSS3?

In CSS3, specifying colors can be done using several methods, including RGB, RGBA, color names, and hexadecimal values. Each of these methods allows developers to define colors in various ways suitable for different use cases.

The method that is not an option for specifying a color in CSS3 is binary. The CSS specifications do not support a binary format for color representation. RGB uses the red, green, and blue color model to create colors by mixing different intensities of these three colors. RGBA extends this model by adding an alpha channel for opacity control. Color names provide a way to represent common colors using easy-to-remember names (like 'red' or 'blue'), and hexadecimal values use a base-16 notation to specify colors.

Since binary is not part of standard CSS color definitions, it is the correct response to the question.