First try to use the Silk Dual Color Bambulab (red-black) and I got one face as expected, red with a bit black and a metal touch, but the other face is almost pure black!
Can we control this? Is it possible to make all the surface appearance like the red one? Or it is just lucky?
Not the way it works with the Bambu’s. If you look at the tip of the filament, you can see one side being red, the other being black. So with a steady, non-twist feed, one side of the print will be mainly red, one mainly black and top and bottom dependant on the direction of the surface pattern.
If this pic roughly shows what is in the nozzle, a left to right line will appear mainly red, a right to left line mainly black. And of course, it all depends on the orientation of the color split in the nozzle.
With a co-extrusion mixing nozzle, you could mix a desired tone throughout from multiple, individually colored filaments.
No, this is not possible. It is like trying to make something black turn red, which is exactly what you are doing. You could slightly improve this for orientation, but you will never get rid of the black.