Thank you for your detailed explanation. The paint job still fresh though when i touch especially at the door bolts. lol
If you look at the engine bay, you can see most, if not all, of the bolts are marked too.
From an engineering perspective, I speculate the possible reasons:
1. As
babylon52281 mentioned, it indicates that the marked bolts have been torqued with the proper torque value. Now, why do we need to mark the bolt instead of torquing each bolt as we fasten it? Well, in industries (non-car), the bolts/nuts will first be tightened lightly. Then the proper torque will be applied manually. Hence, it's not practical to torque every bolt the moment it is fastened. Also, there is a proper pattern to torque bolts, for example, the star pattern for even force, when torquing our wheels. It's not like how typical tyre shop people do it - simply whack from point A to point B. So, there is a risk of bolts not torqued if they are not marked.
2. It allows for easy visual identification of loosened bolt/nut. In every new machine, vibrations may loosen certain bolts/nuts despite the torquing done. As it is neither practical nor possible to check the torque value of every nut/bolt in an inspection, manufacturers can pick a few problematic areas and observe the marking to see any loosening. Hence, I reckon the mechanics are supposed to check key components/bolts for loosening during our 1k inspection.
3. It discourages parts swapping by unscrupulous dealers. If you see that your alternator mounting bolts don't have properly aligned markings, you know it has been tampered with.
and other reasons I can't think of now.