GNOME Color Chooser is a tool that makes it easy to change the colors of GTK UI elements. Since GNOME is based on GTK, this affects the foreground, background and other colors of the UI elements of many applications and the desktop. The settings applied here might override the colors applied by your desktop theme.
Installing it is easy:
$ sudo apt install gnome-color-chooser
It can be invoked from the Dash as GNOME Color Chooser or from the shell as
gnome-color-chooser. The changes are applied on the desktop and applications as soon as they are set. No need to logout of your desktop session.
Curious to know what this tool is changing? I found that this tool writes a
~/.gtkrc-2.0 file. Internally, that just points to a
~/.gtkrc-2.0-gnome-color-chooser where this tool writes all its settings.
Some of the changes I used it for:
Tried with: GNOME Color Chooser 0.2.5 and Ubuntu 14.04