Absolutely, the colors can be changed via the admin panel, you can choose as many as you want and the bar will be generated from these choices. It can be only one color stripe if you want, doesn’t have to be a pattern.
About the donation buttons and widgets on causes, are they optional?
Yes they are, like the progress bar, you can remove the donation button on any causes. You can also decide to keep the button but to redirect it some where else (or directly on your cause page) instead of the paypal donation form. Everything is optional/flexible in the theme.