Comment 5 for bug 919014

This is caused by something introduced in the last cycle, and affects only PCs with more than 4 cores. As a workaround until this gets fixed (I'm working on it, but it could take a while until it gets fixed, commited to GNOME, released in GNOME and in Ubuntu) you can install dconf-tools with the 'sudo apt-get install dconf-tools' command, start dconf-editor, navigate to org->gnome->gnome-system-monitor, and add to the cpu-colors key's value two more tuples in form of (core_number, '#FFFFFF') if you have six cores (the default only has 4 cores, that's why it crashes with six cores). After this you should be able to start system monitor and change the colors of the CPUs from the interface.