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.
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.