The paint will start to get “soft” at around 60 degrees C, it will not “self combust”, i.e.: spontaneously catch fire until it is far, far hotter (a temperature that will have made the saucer burn a hole through the table it is sitting on!). So to answer your question, in my experience of tea lights, their “underneath or bottoms” do not become too hot to sit on the background paint, and that would be fine. I would not suggest, however painting an “oil burner” type thing where the flame of the tea light is directly heating the paint, as this will definitely “burn” the paint, and the paint containing plastic will give off the usual fumes/smoke that are associated with plastic burning which are not good!
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