I think that would actually be `color.value *= (1-amount)`, which makes it slightly harder to explain to the user. However, it does seem like that's similar to what `Qt` does, since they also use a multiplicative factor. If we do want to go multiplicative, though, then we might as well adopt their strategy of using `factor=100` as no change, `factor=300` as 1/3 the brightness, and so on, for consistency. WDYT?