There's really no consistent way to "brighten" colors. Changing the alpha mask would make yellows brighter, sure, but then blacks would become extremely dark, among other unsettling changes. Altering the color mixing algorithm for vibrance would instead render most combinations closer to white than actually deepening or brightening the colors. I tried a bunch of methods early on in development and there's only one conclusion: Factorio trains just are dirty and I'd prefer to keep that aesthetic consistent with manually painted trains.
