Why correct "will" as "shall"? I was taught that, except for emphasis, "shall" means the same thing but is used for first person singular and plural.
I also find it strange that SHOULD is translated SKAL. I would have expected SKULLE or BØR, depending on whether it is a temporal or modal statement.
There is something I don't quite get here.
The question "which color will be used?" means the decision is already taken whilst "which color should be used?" means the decision has not been taken yet.
Does this sentence mean both?
No, as you yourself have clearly described the difference.
Hvilken is which? Should it be Hvad farve skal bruges?
I don't think you can use "hvad" before a noun. I think it must always stand on its own and that "hvilken/et/e" is necessary every time you're using a noun. Then again I am not a native speaker so this is just a hunch.