On the plugin single page (Gutenberg plugin test page), the plugin favorites heart in its unselected state is #cbcdce on a white background which is 1:59:1.

Here's the contrast on WebAIM's checker. This fails for UI components and many people with low-vision will not even know that is there.

Because we don't want to fail Use of Color, it would not be sufficient to just darken the color of the heart. I'd recommend adding a border to it so it has a border and light or no color if unselected and a solid color if hovered or selected.