The Commodore 64 has the following fixed palette of 16 colours:
(Your palette may vary due to tuning of screen, etc.)
What was the reason for this selection of colours? Are these colours particularly easy to realise across a NTSC/PAL signal, or with the oscillator frequencies available in the C64?
(Alternatively, were these colours chosen due to non-hardware factors?)
This email from Robert 'Bob' Yannes indicates that the colours were arbitrarily selectable by the chip designers as a result of the design of the VIC-II.