The lines are based on the colors of various items. Clipping attachments are red by default, so the tree lines are red, but if you change the clipping attachment color then the tree lines will change too.
There is a setting to not use colored lines in the tree, under User Interface
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When unchecked all the lines are gray. The lines can be helpful but are not crucial to using the app. Before Spine 4.1 we did have any tree lines at all! some people may not want the clutter of the lines in the tree, so we provide this setting. If you find the colors don't help, you could uncheck this box.
While you can set the color of many items in Spine, I assume there is still quite a bit more we can do to make Spine more color blind friendly. If you run into other areas that are problematic, please let us know and we'll try to address them.