All the true milk paints are soft and chalky when dry. It will scratch and/or flake easily and some users top coat it to make it more robust. I personally would NOT use true milk paint for an instrument finish. Most of the true milk paints are sold as powder that is mixed with water to use and have a 2 or 3 day shelf life after mixing.
The General Finishes "Milk Paint" is really a true water base acrylic paint that's formulated to look like traditional milk paint but having a tough and wear resistant finish when cured.
The General Finishes "milk paint" used to be available in 31 colors but has been reduced to a dozen primary colors. It is possible to use a few of those colors to custom mix something more to your liking, as I show here. I remember General Finishes having a "custom color mixing" feature on their website.
Basic colors are available from Amazon, but I purchased from an Invalid Link Removed that sold a much wider variety of colors. I don't know if those are still available, but it's entirely possible since General Finishes still sells custom colors in gallon sizes.