This desaturates the colors wherever you drag the mouse. (Do it to the same spot many times, and it will eventually become greyscale.)
This tool offers both freehand and one-click operation.
This tool offers multiple size settings.
See also: 「ほうわ」, Darken, Lighten, Tint, 「だっしょく」, & 「いろのこし」.
Tux Paint 0.9.31