Is cold water better for washing your hair?
It’s often ‘said’ that a cold rinse after shampooing and conditioning can close the hair cuticle and make it shiny. Shiny hair is shiny because it either has a low porosity (strong, healthy cuticles) or because any porosity it does have has been temporarily filled in with hair conditioner or protein. Cold water doesn't decrease porosity or fill in any gaps. So it doesn’t make sense to me, as a hairdresser, that it makes hair shinier.
It's also 'said' by people that hot water can 'open up the cuticles of the hair' when you rinse. The idea that cuticles can move themselves up and down is a bit weird as they're actually formed from dead cells that overlap in layers to protect the hair shaft. If the cuticles are raised it’s usually either because the hair shaft has swollen or because the cuticle is damaged from external forces like heated styling or chemical processes.
Hot water may cause the hair shaft to swell more than room temperature water would. The swelling has the side-effect of elevating the cuticles so they stand up more than usual. So for hair that's already porous, it might become weaker when washed in hot water. Because the cuticles are now standing upright, the hair's natural proteins can leak out and be washed away.
Warm water makes more sense
Common sense would then dictate that hot water isn’t great for washing damaged hair, warm water is probably fine and if you can tolerate cold showers then it is probably less damaging to your hair. But it won't necessarily make it any shinier.
Think I'll stick with warm then. We are in Britain after all :)