After enabling the brush dynamics gradient for velocity, the brush produces unexpected colors.
In case it’s not clear from the screen shot, the fore and background colors are #fddd19 (yellow) and #025a9c (blue). On the canvas, colors like #31f8fe (cyan) and #7f21d5 (purple) are produced. The sprite is in RGB color mode.
Aseprite version 1.3.17 on macOS Tahoe 26.4, opened from Steam. The cursor input device is a laptop track pad.