Hi, ive designed of bunch of lightboxes in cad software and when designing them, the areas that light up will need to be seperate objects so that when bringing the model into the slicer, you can split the model and color individual components on the objects tab. If the lightbox was modeled correctly you will be able to do this, painting is not the method to color these, so try split the model to parts and color on the objects tab by clicking each part on the list.
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