Hello!
I would like to change my white colour to the metalic 0.8 but you can see that it’s doesn’t work.
I trying also with “add” and “multiply” and connect that to the that white colour but this option also doesnt work.
Please help me 
Use a lerp node, the alpha is your texture. that way only the white parts will be metallic.
For this sort of stuff you need a mask. but since your texture is B&W it’ll do.