Dikkie, great question. I'm brand-new here, and fairly new to Graphene and to WordPress itself. Seems like most of what I do know, I've learned from Prasanna, Josh, Kenneth, and of course Syahir. The only thing that emboldens me to speak up now, even tentatively, is noticing that no one else has yet addressed your question.
As you mention, it's possible to change widget colors (by which I assume you mean, the color of the widget title bar). A number of threads on this forum have previously shown how to do this using CSS, adding it either to the theme's "Custom CSS" box or to style.css in a child theme. But as you know, doing it this way affects sidebar widgets on every page, and you want different colors for different pages.
So my suggestion is to try that, placing the title-bar CSS not in the theme options or the child theme style-sheet, but in the header of the page where you want it. Then simply change the color for each page: yellow, lilac, or whatnot. You'd place the CSS declarations between a <style> opening tag and a </style> closing tag. In other words, you'll be creating an internal style-sheet for that page. It will then appear in your page code, just before the page's closing </head> tag. Being located there, it will override styles from any external stylesheet (all other things being equal, like !important specifications, etc.
If the above isn't clear, please ask, and I'll post actual code that you can cut and paste. Also, I wouldn't be surprised if our forum moderators have easier and better ways to do this Like without using a plug-in, which is always a last resort. But this HIFI plug-in has thousands of uses.