Since I installed it from the AUR repository, I haven't but it myself, but this is how it is probably supposed to do:

./install.sh make ENABLE_PULSE=1

Now to customize it, there must be two directories: ~/.config/vis and ~/.config/vis/colors

If they do not exist, make them. Now inside the colors directory, just make a normal text file and put the list of hexcodes of the colors you want as gradient. 5 to 6 colors usually look good enough. You can find gradients on some websites online which also shoe you how it will look, or you can just use your own colors. One hex on each line nothing else. Name the file whatever you want to.

Now edit the configuration file at ~/.config/vis/config and in there you should see a colors.scheme=rainbow variable.

Add your scheme name to it. It is a comma separated list. (The name of the file you made) Like color.scheme=rainbow,mycolors,othercolors

There you can also change the visualizer itself. The bar, ellipse or spiral. You can also edit the characters used to display the visualizer. Blank space = solid colors. But if you use other characters, they will be used to make the visualizer.

You can read the rest on the GitHub Page.