There are formulas to get the colour you want.
If you mix in equal portions you will get a very light silver grey that will not be anything special.
Black can be formed by mixing many colors together.
Black
metallic blue
Maroon
Assuming by 'plum' you mean purple - mixing red & blue together will give you the colour purple.
Black can be formed by mixing many colors together.
Black
Black
Most likely you would either get black or brown.
metallic blue
Mixing any color with black produces a shade of the original color. Mixing a color with white produces a tint of the original color.
White is the visible expression of all color combined as light. Black is a complete absence of light and therefore color. However, when mixing pigment to obtain a color, mixing all of the primary colors will result in a very dark (black like) color. This is because when you are using tools like markers or paints and you mix it all together, your blob of paint has now absorbed every color and no longer reflects color.
Silver or Black, or both together .
shade
Maroon
It may not be possible to get a true silver color, but you can try this: Silver is a metallic color although it would be possible to mix a small amout of black to white as an experiment I tried using some gold paint (or yellow) and extremely little black with mostly white and got a semi shiny gray color.
Assuming by 'plum' you mean purple - mixing red & blue together will give you the colour purple.