Currently, the background color can only be changed per layout by using a specific plasma theme. However, if you have multiple panels (in the same activity), there is no way to set a different plasma theme for each panel individually. Maybe this is quite niche, but to me this is pretty much a dealbreaker
Sorry, just figured out you can have multiple layouts active on the same activity, and how to assign specific panels to specific layouts Therefore (for anyone else finding this), to achieve it, make multiple plasma themes, enable "multiple layouts based on activities" in "configure latte", set a different plasma theme for each layout, assign one panel to each layout, enable the layouts you want in your current activity/all activities, and finally make sure that all panels use their layout's color scheme.