I recently built and deployed a new theme to test out. I used the UI Fabric theme designer, added the primary color (a variation of red) and a darker color for text (a variation of charcoal color) and kept the background color as #fff.
I used add-SPOtheme and added the resulting color palette to my site. When testing, I noticed something weird in the page edit mode. When I hover over adding a new web part, the webparts are highlighted with a dark charcoal type color (when hovering the mouse over) which I thought was really weird.
I inspected the element and found the hex for this color. It did not match any of the hex values that were exported by the theme engine. So now I am not sure where this color came from, and why its being used a hover effect.
I reverted to a different theme and different colors apply then, but I wanted to know if there is something wrong with my initial theme color choices which caused this.
The primary color: #c30631
Text color: #383838
Background color: #ffffff
The offending color when I highlight a webpart option in page edit mode: #1f0108