This is a library and CLI for generating faux-column images as described in Faux Columns for Liquid Layouts and Ethan Marcotte's Responsive Web Design.
You will need node.js and the npm package manager installed to use this tool. With that done, installation is just:
npm install -g fauxfriend
Example:
fauxfriend -l '#525252' -r 'rgba(0, 0, 0, 0)' -p 45.5 faux-column.png
The options are:
-l (required) left column color
-r (required) right column color
-p (required) left/right boundary point as a percentage
The output filename must be specified as an additional argument, and the output format is always PNG.
The colors may be specified using a CSS color. The hex, rgb, and rgba formats are supported. Transparency is supported using rgba style colors as shown in the example above.