I was bleeding my brakes and noticed the paint flaking up pretty bad near the bleeder nipple. Looks like the fluid puddled up at the nipple area near the retaining ring of the captive nipple boot and caused the paint to delaminate.
So I scrapped the delaminated paint away, removed the nipple boot and dried the area really well.
I found some testers silver paint which was part of a hobby kit.
Painted the area and allowed to dry. Replaced the boot and it looks pretty good now. Did all 4 calipers.
Next time I change the brake fluid whether myself or at the shop, I’ll make sure the calipers are well cleaned afterwards which requires the removal of the retaining ring of the captive boot. Also, next time I have the wheels off, I’ll replace the captive boots with stand alone boots to make cleaning easier.
I believe the low surface energy of the boot retainer plus the thin gap initially pulls the brake fluid into the region if it’s not completely removed when cleaning. The fluid causes paint to delamination which creates a capillary channel for more fluid to penetrate. The process continues until the fluid fully vacates the region of the captive ring.
The factory probably should have supplied stand-alone boots with the car considering the painted calipers.