Your English is fine. Often called tulip poplar in the US. It often has green and gray coloring. Wood bleach won't remove this since it's throughout the wood. Wood bleach will also raise grain and can turn the surface soft. You should look for birch, maple or pieces of the poplar that don't have the discoloration.
I agree with the first commenter. If you have a wood of the type and color you want, start there.
However, if you must deal with the dual color lumber you may bleach out some / most of the color with the application of laundry bleach to the colored area then while wet, apply Hydrogen Peroxide directly to the wet bleach areas.
The two chemicals will react! If you do this bleaching process you must take caution to only work in a Well Ventilated area !!
After the chemistry dries you can apply a second time. Each application will be less effective than the first.
After the final application I wash the wood with H2O, (water).
I like the results better than Oxalic Acid.
Experiment first to see if either process will work well enough for your application.
The wood can be bleached with household bleach but it takes a lot of time and patience. The experience I've had with poplar though is it is prone to warp and extensive soaking with bleach may cause you some problems with warpage. It might be better to find a wood the color is more suited for your needs. If you do bleach the wood be sure to wet the opposite side of it with water to help keep the moisture content as even as possible for both sides.
The bleach that removes the natural color of wood is two part A/B bleach. It will leave an almost white color to the wood. Google "two part A/B" bleach for info and sources. Some real paint stores will carry it.
Household chlorine bleach is chlorine bleach around the world. Oxalic acid is available in Europe as far as I know. I don't know about two part A/B bleach but it's the only bleach that will remove wood color.
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