(I think there was a tool that offered this, XKCD style, but I've no idea what it was called)
Early on at my first appdev job, I was showing off all the changes I had made in functionality of the website. The response I got: "make the box bluer"
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".
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.
But for certain fast prototyping it is unbeatable, probably faster than pen and paper once you get used to it.
Mostly worked out pretty nicely and avoided the inevitable digressions into "what shade of puce should the title bar be", but there is no way to adequately express on Hacker News the facepalm I made when someone complained that the sketch/wireframe style wasn't going to work too well for the finished app.