I just used 36 and these are my results:
- purple: 5
- red/orange: 6
- yellow: 5
- green: 6
- white: 3
- light cyan: 4
- blue: 2
- pink: 4
Total: 36
I didn't continue because I already got all 8 colors
Little consideration: I think many people have difficulties in finding all 8 colors because some of them are actually quite similar:
- white and light pink might look the same color under some lights
- yellow and green are VERY similar, if I didn't manage to get 2 yellow and 1 green very close to one another I probably would have thought they were the same color
- light cyan: I called it cyan but it probably seems like a light green or light blue on different screens.
I did the experiment in the tunnel of the female undead merchant, which is dark enough and close to the merchant
I got only 2 blue but I can't tell if that's the rare color, because I only used 35 stones - not quite enough for that. I was just interested in finding out if there were 7 or 8 colors
Anyway, sorry for the bad quality image, I actually had to take a pic with my phone, so... the stones colors are off, but the particles are pretty accurate so I worked with those. Difference between yellow and green is more obvious in the highlighted area