The FX-CSS guide lists two color-functions[1], "derive" and "ladder" and reasons that the help to "[enable] a color theme to be specified using a single base color and to have variant colors computed from that base color."

In that vein, I would like to propose a sister-function to "derive":
Called "transparent", it takes a color and a percentage as its argument and returns the base color with its alpha value set to the percentage.

transparent(red, 0.5) creates semi-translucent red
transparent(#00ff00, 0.25), creates a faint green

A function like this would help to further reuse in fx-css and thus help to achieve a color scheme based on few base-colors.

[1] http://docs.oracle.com/javafx/2/api/javafx/scene/doc-files/cssref.html#typecolor