It seems that if you polish the inkstick slowly and gently you get a different color than the color you would get if you polish it strongly. Am I correct?
Yes, and the reason why it happens is because of a difference in the size of ink particles. If you polish an inkstick forcefully the inkstick will breakdown in larger particles. If you polish the inkstick slowly and gently the inkstick will breakdown in small particles. Smaller ink particles spread easily on paper. So you can get softer and gentle edge with smaller ink particles, I guess.
@cisp360 Thanks for watching my video! 😃 You are right. As others have explained in detail, the way the ink is handled changes the expression. Gentle handling is the basic principle.