RGB to colormatrix not accurate

[illustribe]

Hello, I'm trying to apply the fecolormatrix values I've got from your website (andresgalante.com/RGBAtoFeColorMatrix/) to get a solid color of the RGB values I've entered using a basic solid color to start with. I've tried with black (doesn't work), with white (too light) and with 50% gray (it's not the right value), but haven't reach the exact source (original color) to begin with to end up with the desired color (the RGB values I've entered in the converter). I guess it's some kind of neutral gray that would do it, but I wonder if you would know that already, so I don't have to test all of the shades of gray...
So far I've come the closer with #C8C8C8, but it's not there yet. I'm guessing the converter has a starting point (basic color) to begin converting from (abstractedly speaking), so would you know from which source color I could transform it to my desired one?
Thank you!