I looking for the right command in c# to do this:
Maybe it gives a other way to do this?I would like all white as transparent so we can see the bellow layer (wooden maple)
Since your image is pretty much black and white, why not convert to Black and White (1-bit) then set the fill color to nothing. This would allow you to see the wooden maple layer below.
Here is a little example. You can see I have my Black and White (1-bit) Bitmap selected. And since I have set the fill color to nothing I am able to see the cyan shape behind it.