You’re absolutely right that you could consider this a hack, however we prefer the default color of the app to be orange not blue, that’s a design choice. Just like Apple choose blue as the default color for selection on macOS. 99 out of 100 people stick with blue because it’s the default not because they particularly prefer blue over any of the other choices, which is why we use our “default” color if you have the system set to the default highlight color. Unfortunately this comes at the cost of not being able to force Agenda to be using the blue color.
Having said all this, we’ll try to add a hidden preference to override this choice in the next update.