"Rinse your hair with ice cold water to close the cuticle to make it shine."
Ever try it?
And then there's the opposite - that very warm water makes your hair's cuticles "open up."
I don't get along with the notion that "cuticles open and close" like doors or window blinds because it implies that the cuticles have some method of creating their own motion. They don't. If a cuticle is "open" it's actually elevated from the hair surface as a result of the whole-hair swelling in water or other solutions, or it is damaged by chemicals or friction and broken but has not yet torn completely off.
I had to test it myself and I'll admit my tests are not complete (if I wanted to spend all day working with 50 to 100 hair samples and perfectly controlled conditions that would be ideal - but I'll leave that to people who get money to do this sort of thing).
The "Cold Water Makes Hair Shine" notion is just that - a notion. It's an assertion which is difficult to measure on your own hair. The eye can be tricked - I mean, if you just put your body or head through a cold water rinse to make your hair shine, will your assessment of the shiny-ness of the end result be biased by your desire for it to work? It would be difficult to avoid not seeing what you want to see.
Shine breakdown: I'm going to be looking for a change in porosity because "closing the cuticles" implies reduction of porosity. Shiny hair is lower porosity hair or hair which has had it's porosities temporarily filled in with conditioners or emollients and proteins.
There is another element of shine which has less to do with cuticles and porosity. When the hair as a whole has a greater reflective surface, hair shines. In order to do that, hairs need to line up neatly with each other. This has more to do with curl pattern, the "slip" in hair products you used, whether you wear your hair is worn straight or smoothly coiffed or tousled or worn naturally curly, and whether your individual hairs are smooth or textured.
There are some standard facts about hair and oils and heat that apply to this question:
1) Warm to hot water assists in removal of oil and soil from hair - including sebum - with but also without detergent.
2) It is more difficult to remove oil and soils (dust, dirt) in cold water.
3) Heat: Dry heat causes cracks in cuticles which tend to develop into chips and ultimately break away as they wear and weather. Hot water heats in a different way which is not necessarily well-studied in hair because hair product manufacturers are more interested in shampoos and conditioners and styling products and curling or straightening irons and blow dryers than hot water.
4) Warm water (heat) speeds up reactions such as diffusion of solutes (proteins, amino acids) in and out of hair; heat liquefies and "thins" more solid or more viscous oils. Things happen faster in heat, in this case oils and proteins are lost from your hair or removed if that's what you're after.
For this project I chose low, normal, and higher porosity hairs and photographed them in room temperature water, in very cold water and in very warm (shower temperature) water.
Cold Water (roughly 45-50°F / 10°C): There is little difference in the hair between room temperature to cold water. Some hairs seemed to have a more dramatic swelling reaction in cold water than room temperature water. No hairs seemed to show a reduction in porosity which would be indicated by flatter-lying cuticles when viewed on the "edge" or by decreased swelling in water compared to room temperature water.
Warm-hot water (roughly 100°F / 38°C): In warmer water, some hairs are evolving bubbles and showing increased swelling. This makes sense because heat speeds up reactions and makes oils more liquid. Proteins and oils can diffuse in and out via porosities in the hair more readily so the very warm (hot) water speeds up any swelling with water that will occur. We see bubbles evolving around hair when water is moving into the hair and things inside the hair are moving out. So maybe the hair is more porous in the warm water, and certainly porous hair is especially vulnerable in warm to hot water.
Cold water does not decrease these hair's porosity so it should not make the hair shine more. It may be detrimental to some people's hair.
Warm water may cause hair to swell more rapidly than room temperature water. Swelling in hair has an ultimate side-effect of elevating the cuticles so they stand out more than usual. This might cause hair to act more porous temporarily, but not to the extent that soaking in concentrated detergent, acids, or bases does. Hair that is already porous may become more vulnerable in warm to hot water. So yes - that does sort of make cuticles open up - and I'll add that while they're open, they're losing their native proteins and other goodies which are rinsing down the drain.
Hair returns to its original state when its temperature returns to normal.
My impression: Keep out of the really hot showers and save "ice-cold water over the head" for the coaches of winning football games. Or would that be Gatorade? Or would that be basketball?
If you swear cold water works for you - go for it. I don't have any love for being in ice-cold water. It turns my toes blue. I could not find any research supporting the cold water rinse it (and I probably never will - it's not exactly a marketable product). I could not produce lower porosity (therefore shinier) hair with cold water. That satisfies my curiosity.
This is a kinking, curly hair (curl is pencil-sized, kinking is fairly close together) of low-normal porosity in the room temperature, cold, and hot water treatments. In the "before" room temperature photo at left, the cuticle lies closely to the hair. In the cold water photo (middle) the blue arrows indicate elevated cuticle scales where the hair is probably swelling during the shock of the cold water. At right the blue arrows indicate where the hair is swelling in the hot water.