Usage: background_color(color)
The background_color filter sets the background layer to the specified color. This is specifically useful when converting transparent images (PNG) to JPEG
color- the color name (like in HTML) or hexadecimal rgb expression without the "#" character (see https://en.wikipedia.org/wiki/Web_colors for example). If color is "auto", a color will be smartly chosen (based on the image pixels) to be the filling color.
The original image is:
http://localhost:8888/unsafe/fit-in/300x300/filters:background_color(blue)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fdocs%2Fimages%2Fdice_transparent_background.png
http://localhost:8888/unsafe/fit-in/300x300/filters:background_color(f00)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fdocs%2Fimages%2Fdice_transparent_background.png
http://localhost:8888/unsafe/fit-in/300x300/filters:background_color(add8e6)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fdocs%2Fimages%2Fdice_transparent_background.png