Another attempt to measure the 'real' colors of a Commodore 64.
For this I used three Commodore 64s. Well, in fact, two real C64s and one Ultimate 64.
I took measurements using the S-video input of a bt848 capture card at different saturation levels. Then adjusted the saturation down from 100% until all colors were within the RGB gamut.
For the classic C64 'brown' (9) is the most difficult color in a sense, since even at moderate overall saturation its blue component becomes negative.
On the Ultimate 64 light green (13) is the first color to become out-of-gamut at high saturation levels.
Once the ideal saturation level was determined, I averaged 1024 captures of a reference image containing 16 rectangles of all colors, then for each color computed its average value in each of those rectangles.
An interesting phenomenon that cropped up during this was that the C64 renders colors incorrectly: there is a significant phase difference in the color signal between odd and even scanlines. Normally these differences are cancelled out due to the way the PAL color system works. If you zoom in on the reference images below you can see the difference between odd and even lines. (This doesn't apply to the ultimate64 of course.)
The samples were taken at default brightness setting, so white is either too bright (ultimate64, board #1), or too dark (board #2)
These are PAL boards, so I may have spelled the word 'color' incorrectly.
Jump to: board #1, board #2, ultimate 64, scope measurements.
Addendum (September 2020)
The method of 'adjusting until all colors are within gamut' results in very desaturated-looking colors. Also I did not take into account that I really had to adjust the contrast setting as well. I then took another approach: base the colors on what is actually seen on an oscilloscope. The problem of course here is that the chroma voltage levels are wrong (C64 does not output s-video really), so for the chroma values I cheated a bit and used the readings from the composite output.
board #1 (adjusted)
June 2020, updated August, September 2020