Update (28-Feb-2018): It's now possible to define up to three color "steps" for the progress bar.
For example, Red for 0%-50%, Yellow for 51%-75% and Green for 76%-100%.

The new tool dialog looks like this:

And the resulting progress bars look like this (notice the different colors, based on their values):

If you want to "spice up" your forms, you can use this tool to convert your percentage fields to a graphic element, a progress bar, that updates "live", just like any other field.
You can select the bar's color, as well as whether or not to show the percentage value.

Here's the original, boring, form:

And then you run the script, select your percentage fields...

And the result is...

You can even download the actual PDF file and see how it works for yourself, here.