I think the way to go is to create a mood board / app style guide / (whatever you like to call it) that shows the design of elements. So you will have the design of buttons (all states), fonts, font sizes margins, and maybe even only one fake page design.

When you present this next to the interaction design customers can understand what is going on and most can imagine what the end product is going to look like.

And imho it's also a managers job to guide the customer away from (for that moment) distracting remarks like 'make the box bluer'. You can simple fix this by saying something like "Ok, we will note your remark about the color".

A lot of larger companies have responses just like this. I was in a meeting yesterday where we talked about a progress bar for 30 minutes.

Years ago I decided to demo everything in HTML/CSS. I find that I can write web code faster than I can whip something up in a prototyping tool. The demo can also be viewed in a browser. It's not 1:1, but it gives me a lot more control to make things bluer.

I have been using Balsamiq mockups (Desktop) for years and the lack of interaction design is still a huge drawback that sadly won't be fixed. Which means that when you are showcasing/previewing your mockups, users can't really interact with them as you would with usual UI widgets.

But for certain fast prototyping it is unbeatable, probably faster than pen and paper once you get used to it.