First and foremost—not all marine paints are created equal, not all paints share the same type of colorants, and not all universal hardware store colorants are compatible with your paint.
1-component paints: These are the most common paint systems used by the DIY boating community, and customers are frequently looking to customize colors, or match existing colors. To tint 1-component paints, it is imperative to contact the manufacturer because they're asked this question regularly. For most circumstances, they'll usually recommend blending two or more colors in the same paint system to create the desired color, but for more extreme cases, they'll be able to spell out your options. Many customers go straight to the local hardware store and add whatever pigments are recommend there. Much of the time, this really works out well, but from the number of unsatisfied customers, it's strongly recommended to contact the paint manufacturers first. As for adding metallics and other effects, we also recommend talking to the manufacturer before adding any effects because these special effects look better when they're suspended in a clear coating.
2-component paints: These paints should not be adjusted, tinted, or changed beyond the specific directions from the paint manufacturer. Many of these paint systems can be customized by the manufacturer, and some manufacturers offer effects such as metallics, pearloid, and clear coats. Do not try to mix other paint brands or hardware store-grade colorants with 2-component paint systems.