Saturated means the amount of pure color in your value, 100% C is fully saturated blue. This is not usually applied to greys but in this case it can be helpful:
We have a fully saturated grey for active backgrounds and this screened back grey (screened means greater transparency) for inactive areas.
Point being, get your terms and your arguments arranged. Why is this grey here and why is this grey here?
For highlighted features we use this bold grey and for background areas we use this neutral grey.
For the foreground we use this bright saturated grey and for the background we use this neutral grey.
To draw attention we have this tinted grey and for neutral areas we have shaded grey (tint means a color value with white added and shade means color value with black added).
If they ask why then start in on stuff like this:
Why? because design theory prescribes driving attention with color, placement and size. These greys are used to bring the users attention to the places we need first. Sophisticated users are expecting certain patterns and we need to provide these patterns through application of design theory. Color theory applied to cognitive response is an interesting and rich field, I can recommend you books and courses if you want to know more. Of course these greys are neutral on the spectrum but the subtle differences are used by designers to distinguish different hierarchies of neutral, you've got de-activated grey, background grey, shadow grey, preview grey. Believe me people I would like to use more greys than these but I'm limiting the palette to really focus the message...
and on and on like that until they tell you to stop. The point being, the reason for these color decisions have to do with design stuff which your background and experience uniquely you to answer. If they want to see into the process then overwhelm them with wealth of actual reasons you use when making a design decision, referring to the body of knowledge which describes our craft. In other words, I feel it should be like this because I'm the designer and this is what good design is.
Defend your decisions by being resolute on best design practices, but willing to capitulate if they choose the bad design.