The following tutorial was tested on Ubuntu 8.04.1 with GNOME 2.22.3. Anyway, we are interested in the gnome-mag package version, so you should check yours right now! How? Open Synaptic (System -> Administration -> Synaptic Package Manager) and search for gnome-mag. You will see the version number on the 'Installed Version' column.
OK, so let's get down to business and recompile the gnome-mag package with colorblind support. First of all, it is essential to install some development tools, if you don't have them (most people don't). Open a terminal (Applications -> Accessories -> Terminal) and paste the following command:
sudo apt-get -y install build-essential libxv-dev libxfixes-dev libxdamage-dev libxcomposite-dev libxrender-dev liborbit2-dev python-object-dev libatspi-dev libcolorblind-dev libgnome-mag-dev python-gtk2-dev python-gnome2-dev libgnome-desktop-dev gettext autoconf automake libtool gnome-common
Don't close the terminal window yet!
Suppose the gnome-mag archive was saved on your desktop. Extract it, enter the folder and type on the terminal the following commands, one by one:
sudo make install
sudo make install
Note: Don't forget to change the version number from the first command according to your installed package!
When the installation is over, log out! When you're back, right click on the panel and left click on 'Add to Panel'...
That's it folks! Right click on the Colorblind applet to access the preferences, from where you can choose a default filter and change the access keys (defaults are Shift + Ctrl + c to activate the filters and Shift + Ctrl + n to change them).here and here or search Google Images for colorblind!
Editor's note: It appears that the filters don't work with Compiz Fusion, so you should disable it first (right click on the desktop -> Change Desktop Background -> Visual Effects -> None).