I wandered across this Thingiverse “Thing” today: it basically is an STL to print filament. The trick is that one can change the color of filament printed between layers, and when this filament is fed back through on a new print, the different colors melt and blend together to make a custom color.
Note that this is different from the color-change “rainbow” filament which prints in multiple colors from a single spool by changing colors every few feet: this lays down two or more colors into a new filament with the color change across the filament. When remelted, the colors blend to make a homogeneous color (in theory at least - I haven’t tried it).
Dark Blue + White = Light Blue
Red + Yellow = Orange
Black + Anything = Mud (probably)
An interesting take on hard-to-find filament colors, or when you just need a small amount of a specific color for a project (assuming you have the component filaments for the custom blend).