As far as I know early German vehicles were painted straight dark grey which is really dark. There is no way to know that the halftrack in the above image was painted with the right color. To my knowledge no blue grey was used.
Now from a modeling point of view people do their shading of the dark grey different ways and you will get a dozen different opinions on what's right. I prefer to add a little blue to my grey, but others shade with lighter shades of grey, and still others just add dust and dirt effects. In the end it is really up to you which you like and go that direction because I don't think any of these options are wrong. Here are some examples from here on the site of different takes on dark grey German equipment.
This one is mine.
This one is by Laura
This one is by Andy
All are a little different and there are many other examples. What it boils down to is you do it the way you like.
B)
Now from a modeling point of view people do their shading of the dark grey different ways and you will get a dozen different opinions on what's right. I prefer to add a little blue to my grey, but others shade with lighter shades of grey, and still others just add dust and dirt effects. In the end it is really up to you which you like and go that direction because I don't think any of these options are wrong. Here are some examples from here on the site of different takes on dark grey German equipment.
This one is mine.
This one is by Laura
This one is by Andy
All are a little different and there are many other examples. What it boils down to is you do it the way you like.
B)