Hello,

You can save all the surface assigments in one file. See online help here.

From Allplan 2020, you can directly apply a transparency value to all objects or only to what has been selected in the Objects palette.

If you apply a surface by a color, you can do it like I explained in my first answer. Try it and you will see.

Your elements must have a surface by the color, not applied on the element.
When you apply a new color on the elements, they lost the previous parameters and there is no possibility to store them somewhere.
But, if you use the surface according to the color, you can store the settings and you can switch between "surface by color" and "surface assignment" to the elements.
If you do that, when you applied the transparent surface, you can applied "no" and come back to the previous surfaces.
See the 3 steps in the attached pictures:
1 - You assign surfaces according to the color of the element.
2 - You change to transparency.
3 - You remove the assignment and get back the surfaces by color.

This works not with color by layer, is that right?

It works with color by layer too.
There are two cases:
A - With an element with color by layer, you can assign a surface like in my step 2 and later you can do the step 3 to get the initial surface.
B - With an element with its own color, you can assign it to color by layer and it gets the surface assigned to the color of the layer, and later you can remove the color by layer to get back the initial color and surface.

Wwe have layers with the same color, what can I do?
(The elements have different materials)