Custom guides can be added, and their color changed but you can't customize the guides' color or add multiple guides with different colors. This would be helpful to differentiate various guides for various purposes within a picture.
When was the last time you were affected by this lack of functionality, or specific tool?
This is not a current feature available, so it is happening now with the Guides Tool
Current workaround
Are you using any workarounds or other solutions to achieve your goals in Capture One?
There is no workaround. You can alter the color of the guides universally within a file but the only way to have multiple colors on an image would be to have multiple variants of a picture with guides of various colors applied to each cloned variation.
