Record a pattern rather than a diagnosis
Note the time, which face receives sun and whether the symptom disappears after the surface cools. A photo taken from a safe stationary position may help. Do not place yourself under the door, touch hot surfaces or force another cycle for a video.
Thermal bowing is one possible explanation
DASMA describes how a temperature difference across certain bonded insulated sections can cause bowing. Color, exposure and door design can affect that behavior. The pattern alone does not rule out damaged hardware or a different installation problem.
Bring the selected finish into the assessment
Keep the model and any repainting history available. Ask a professional to compare the symptoms with manufacturer guidance before changing hardware or controls. Additional reinforcement, track adjustments and force-setting changes should not be improvised as a response to afternoon binding.
Your questions
Does afternoon rubbing prove the door has thermal bowing?
No. Timing is useful evidence, not a complete diagnosis. The door and its installation still need assessment.
Would painting it a lighter color fix the problem?
Do not treat repainting as a guaranteed repair. The model, finish instructions and actual cause need to be checked before changing the door.