I never do cold water rinses. Pay close attention to the bolded part:
Cold rinses don't make hair any more shiny than lukewarm or hot ones do, so why put yourself through the suffering? Besides, cold rinses have a negative effect on hair — they constrict blood capillaries in the scalp (tiny blood vessels that deliver nutrients and pick up waste products in return). If the capillaries are to do their job properly, they need to be allowed to operate in a non-constricted state.
Shampoo residue can dull the appearance of hair, so rinsing hair thoroughly after washing it can help lessen this dulling effect. But it makes no difference what temperature of water is used to wash the shampoo away.
Sure! But only
temporarily. When your scalp warms itself back up, the capillaries re-open just like everywhere else on your body. Your logic suggests that capillaries permanently die from any amount of cold at all! If that were true it would be life threatening to sit under an Air conditioning unit for 5 min. It would be taking your life into your hands to live in ANY of the northern states during winter.
Besides that, the scalp is the most vein/artery rich part of your body there is! There are so many arteries and veins and capillaries up there that no matter how much cold water you put up there your body WILL find a way to get blood to your scalp during the few min your capillaries are shivering.
Cold water rinses are FINE. And TOTALLY safe. Don't tell people that untruth please.
The temperature of the water
DOES make a difference. Do a little research on temperature v. expansion and constriction.
Cold temperature forces the cuticle of the hair to shut down close up and seal completely therefore making the hair appear shiny and softer looking.
Why do you thing HEAT styling damages hair????? Because the heat forces the shaft open and therefore weakens it.
Like wooden doorways in summer vs. winter. The heat of summer causes the wooden door to shrink and warp. And so when you close the door. It no longer fits flush to the doorway. However, in winter the door expands and you have a tight seal between the door and the doorway.
Temperature, PH, and manipulation are three HUGE factors in how shiny and healthy your hair is!