krammor Thanks for the additional suggestions! Unfortunately, even if we were to use the Ctrl key, it would not work very well in terms of consistency with the operations in the other views. Holding down the Ctrl key and left clicking for multiple selections is a unique operation that is not available in any other view, so it may not be intuitive. The Ctrl+right-click combination is used to pan the Dopesheet view horizontally and to expand and collapse the timelines of a bone by clicking on its name.
You are right that saving a selection also involves the hassle of saving that selection first, but I think this operation is very clear. It's hard for users to notice features that are only available under certain conditions, so I think a feature that is easy to understand and reliable to use would be better.
As I was thinking about this operation, it occurred to me that it is not really a selection issue, but rather a difficulty in managing color changes for multiple slots in the first place. In fact, the issue of adding a more manageable way to change the color of multiple slots has been on our roadmap for some time: EsotericSoftware/spine-editor347
We have an idea in this ticket called SlotColorConstraint: when the color of a particular slot changes, the colors of the other slots that are constrained also change. Whether or not such a management method is actually a good idea is still up for debate, but I think your problem would be solved if there was a way to combine the timelines of multiple slots.
